To preserve environments as they exist without (further) human interventions. There are plenty of examples of that right here on Earth, let alone outer space.
Personally, part of me hopes humanity never figures out how to get away from Earth and spread to other planets. An untouched rest of the universe seems far more valuable than one tainted with humans, at least to this particular jaded homo sapien.
> An untouched rest of the universe seems far more valuable than one tainted with humans
This seems like a silly worry given the rate of expansion of the universe. We couldn't "taint" the universe even if we wanted to. At least nothing outside of our galaxy (but more likely outside of just our solar system).
The universe is essentially infinite. With essentially infinite planets, moons, stars, solar systems, and galaxies. Humans are unique (until proven otherwise). I say let the humans have their fun in their 0.000000000000000001% of the universe. In both best and worst case scenarios, there is still plenty of "untainted" universe left.
There are an estimated 10^22 to 10^24 stars, or in the many sextillions range. So really, don't worry, we wouldn't have the capability to taint any significant amount of the universe even if that was our sole purpose.
Personally, part of me hopes humanity never figures out how to get away from Earth and spread to other planets. An untouched rest of the universe seems far more valuable than one tainted with humans, at least to this particular jaded homo sapien.