Many fictional spaceships of all kinds are known to have external lights. Do they even need them? Outer space is lit by the billions of stars and galaxies, so why are there external lights on fictional drive ships, war ships, cargo ships, etc.?
Would these external light sources be useful, at least?
Or do they just risk a facility/ship wide brownout or loss of function from the generator(s) if hit or destroyed?
Short answer is Yes; external lights are VERY necessary on spacecraft, especially if you have to do external maintenance, and especially during interstellar flight where the light from the sun is not overwhelming the lumen count.
Two things about stars; they're very bright, and they're very far away. The reason you don't see stars during the day is because our sun is so close that its light overwhelms the atmosphere and makes it impossible to see other stars. At night, even though we can't see all the stars that are visible in space from the surface of the earth, it's not because the atmosphere is filtering out light, it's because it's diffusing it a little. In short, space is still dark.
The picture above is of Apollo astronauts taken on the surface of the moon during the moon's daytime noting there are no stars in the background. This is not because the entire moon landings were shot on a sound stage, it's because the light from the sun meant that the exposure setting for the pic had to be set at a level that wouldn't be powerful enough to pick up the stars in the background. The light from the stars is impressive to look at, but doesn't contain the lumen count needed to actually see much by.
So, you'd need lights to see things (especially in crevices) on the surface of your spaceship if you were doing repairs and you might even need it to detect space debris in your path as well in some cases, although Radar would probably be better for that.
To address additional comments
I'm assuming current technical knowledge of physics, so a sub-luminal interstellar ship; either generational or sleeper (probably sleeper because of resources). Using lights as navigational beacons like seaborne ships will be useless because at a significant percentage of light speed, by the time you see the other ship in your path it's too late to change course or react in any way. the only exception to this would be rotating lasers (the lighthouse effect) that would 'blink' from your perspective. The real problem is that given relativity, the light would be invisible to human eyes, and probably act as lethal cosmic rays so not a good idea in any event.
That said, small power LEDs could be left on in perpetuity around the ship as fixed light sources for those difficult knooks and crannies in the hull. There's a good reason to leave them on permanently (other than the obvious real time closed circuit video for monitoring) - cold. Space is cold[Citation Needed] and electronics don't generally play nicely with cold. So, leaving them on full time means that they can sustain their own operability with the heat from the energy they use. Also, leaving them on full time means that if the light isn't on, then something has gone wrong with it directly rather than your work experience bridge officer forgetting to turn on the power during your EVA.
Fixed lights are always ideal for hull inspections (which would need to be conducted on a regular basis because of the risk of dust strikes and the like at such speeds) and if you come out of a hatch at the rear (say), then being able to look down the length of the ship and see the lights on at regular intervals gives you a good long distance view of the ship that your suit light wouldn't be able to deliver. Assuming rails for tethers et al instead of some form of mag boot (which causes issues if your suit power fails) then lighting all over the hull is critical during EVAs so that if you jump from place to place using the tether to give you angular momentum, you can see ahead of you to where you want to land.
Atmosphere-entering spacecraft (Shuttles, space planes) visiting Planet Earth must carry the same navigation lights as other atmospheric transport - white light forward, red to Port, green to Starboard - inherited from terrestrial COLREGS, originally the Steam Navigation Act of 1847, via that planet's winged atmospheric transport craft.
Actually this answer was intended to be humorous but what do you know ... satellite navigation lights really exist!
Anyway, never pass an oncoming spacecraft green to red, in this planet's jurisdiction. And be aware of local regulations in every system you plan to visit - especially where different visible frequency ranges have evolved, according to the emission spectra of the local stars.
Edited to add a photo of actual starboard running light on actual spacecraft (Crew Dragon) during docking manoeuvre. Photo attribution : I'm guessing NASA.
Those lights exist for one terribly important reason only, and that is to make the ship visually appealing for film and television audiences. Same thing goes for the sounds these ships make while flying.
In reality, when you're sitting out in interstellar space, your colorfully painted ship is just going to be a black shape in the blackness of space. Its running lights will only be visible for perhaps a few kilometres.
Now, those lights will be useful in spacedock. All the ancillary vessels (shuttlecraft and so forth) will need to find the airlocks. Spacewalking crews will need to find important hatches, couplings, conduits and so forth.
A spacedock itself will also need exterior lighting. If it's a large facility that ships enter, the doors will need to be well lit, for example. Standard warning lights on spires, towers and so forth.
No, we do not need external lights except for human guided close-approach maneuvers.
Movies look cool when you can see the spaceships. A black ship in black space isn't good cinema.
External lights can't do anything except make the spacecraft visible from further away to visible light sensors (like eyes).
Yes, they would waste energy, but there may be plenty of energy to spare, depending on what you assume for the drive system.
Useful only if the light was needed for something (tautology in the answer). They would only be useful for short-range, human or vision controlled maneuvers such as docking.
Assuming there was enough energy to run the lights, I doubt that lights would make a brownout situation much worse, if the main power were lost.
It's hard to say what we, as human, would do if we had the ability to do galactic-level operations. We might still light spacecraft, especially to show off the external company logos and craft "tail numbers", if the terminal times (launching and docking) were large compared to the transit times. IMO, we wouldn't bother with external lighting.
For ship-to-ship docking, close proximity manoeuvres, landing in bays or on planet surfaces at night, surveying the hull for damage, shining a spotlight on obstacles or targets, and all the other things ships get up to when they aren't crossing the interstellar void a jillion kicks from anything else, lights have their uses.
outer space is lit by the billions of stars and galaxies
For some definition of "lit". Something that's being lit by the billions of stars and galaxies is darker than things are at night on earth. At night on earth, earth is lit by the sun (reflected by the moon and/or scattered through the atmosphere), and by artificial lights. Also, things in space are usually a couple million kilometers away, and moving at a couple kilometers per second relative to your own speed, which makes spacecraft-sized objects very difficult to spot even with external lights on.
are external lights capable of doing something else than illuminating a small area around the light source?
Would it waste energy if installed?
There is no drag in space, but there is mass. Accelerating and decelerating the additional mass of the lights (and the wiring) requires some additional energy. In many applications that will be negligible - a notable exception are nano- and microsatellites.
Would these external light sources be useful, at least?
At worst, having lights prevents you from getting fined by the space police, at best they save your live.
Navigation lights to be seen, to avoid accidents and comply with regulations.
If you want to investigate asteroids, debris, or other ships, then you'll also need some searchlights. Even if you're quite close to a star, every object has a side that is unlit. Once you're between stars, all sides are unlit (unless we're talking binary star systems).
If you want other vessels to dock with you, you need additional lights to mark the docking area.
In case of equipment failure, you also want signal lights for minimal communication.
Interior lights + windows.
Lights for in-flight repairs.
Also, the marketing department wants to make some glamour shots of your vessel.
Or do they just risk a facility/ship wide brownout or loss of function from the generator(s) if hit or destroyed?
Not if installed correctly. Remember, if you install your own space lights, the installation must be verified by a certified space electrician.
This isn't a complete answer, but too many thoughts to be a comment.
I think of the lights would be used at more of a slow speed, so that the spaceship can see the area immediately around it. Small debris or other ships that might not show up on other spectrum scans (like thermal, etc,) could be seen at low speeds. Even if they are seen at high speeds, the reaction of course change may not be quick enough to make a difference, so exterior lights may not be necessary. Lights are also used to show position to others, so even if they aren't useful to you, they may be useful to others.
In combat, exterior lights could give away your position, so they may not be wanted. Then again, they may show friend vs. foe. At the beginning of the movie "Courage Under Fire", a tank battle is happening at night. A commander inadvertently shoots and kills a friendly tank in the dark, so he makes a command decision to turn on the exterior lights of all the tanks and prevents it from happening again. Also, other factors like engine heat and launching missiles or counter measures would give away position, so lights could go either way.
All lights, whether they are in a building or a vehicle, should be on a fuse or circuit breaker to prevent black/brown-out conditions due to failure. They should also be wired parallel, so a failure in one doesn't prevent others from working.
The external ligths on space ships serve the same role as ligths on curent freighter ships, they are big asf and if you pilot a smaler thing not to hit them mostly xD. As for our future i'm willing to bet on yes ... RGB ligths for the space mases !
In deep space, light is just another frequency of RADAR.
Like RADAR, it would have considerable throw range and good return. And unlike RADAR, the frequencies are ones humans can literally see. That greatly simplifies data analysis, because they can call it up on a display in the same frequencies and look at it directly.
How do you suppose it is that the Enterprise can say "closing to visual range" and then "on screen"? Visual range is infinite, what range are we talking about? The range of the Enterprise's lights.
Looking directly through long-lens optics would also be an option: but not a recommended one, because if you accidentally tracked across a star or the other ship's own lights, you'd be blinded.
