I'm surprised nobody's argued this way yet, but I think it's crucial:

### Laser ≠ collimated light beam

### Laser = repeated [(coherent) **amplification**](https://en.wikipedia.org/wiki/Stimulated_emission) of light

Yes, we use the stimulated emission mostly for generating light (often _not_ collimated BTW, it's the coherence that's most important), but also [for amplifying a given light source](https://en.wikipedia.org/wiki/Optical_amplifier). And that would probably be the biological motivation for evolving tissue that can act as a gain medium: to sense very weak light stimuli, e.g. luminent predators or prey.

This would probably happen in the eye of a cat-eyed creature, between the retina and the [tapetum lucidum](https://en.wikipedia.org/wiki/Tapetum_lucidum). The latter is already a reflector, which is the other thing you need for a laser. Once all of this is in place, it would be advantageous for another semi-reflecting layer to develop in front of the retina: though this would block some of the incoming photons before they even get to the retina, it would also send the ones that are already there another time through the amplifier, which generates even more photons... and so on. This is most efficient when the spacing between the mirrors is tuned to the frequency of emitted light: that's how a resonant laser cavity works.

This way, the eye would become extremely sensitive. The downside is that it also becomes very nonlinear, and is readily saturated – it can only detect that there _is_ light, but is bad at making out details. Thus, our creature would evolve an asymmetry, with only one eye developing ever-increasing gain. The side effect of this is that the one eye would actually _light up_ in response to an incoming source. This light then would reflect off the predator and could thus be used by the other eye to see more details. You've developed an automatically-triggered torch!

At this point it's clear that ever stronger pumping of the gain medium is an advantage, to make the flash-light more useful. And whilst the predators or prey haven't adapted to this, they would probably be confused and/or blinded by the flash, giving you time to escape or catch them, respectively. This further incentivises evolution to make the laser stronger. In the end, it might get to quite formidable power (evolution is great at optimising quirky features to surprising strength – consider hagfish, chameleons or pistol shrimps), probably not enough to hurt anything but certainly to blind it. Our creature's laser-eye would by this point not be useful as an eye anymore itself (because every firing scorches its own retina), but that's not important anymore because the other eye has been hugely upgraded through the laser.

Clearly, all of this would be most likely to work out in a deep-sea environment, where there's _never_ sunlight and lots of luminescent animals.