If you went to a planet or moon with enough water there already in some form like ice or something like that under the surface:

Then you have possibly enough water to create oxygen to breathe there even if there isn't enough oxygen in the air there to breathe happily. So, water in the form of ice likely would be a godsend wherever humans go from here out in space.

If there is water that means through solar power (if you are close enough to the sun) or another sun in another solar system that you could through electrolysis create both hydrogen for fuel for fuel cells or rocket propulsion or and at the same time create oxygen to breathe as well if you had solar electrolysis available from enough sunlight to do this.