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.