First, we don’t know Earth is the only habitable planet. However, until we have some proof, we have no way of knowing for sure. That’s why we’re searching so hard for extraterrestrial organisms.
Second, who says the universe has to have a use? It just is. Similarly, you could say what’s the use of living if you die in the end. That kind of view doesn’t do anyone any good. Just go with it and enjoy the exploration and discovery life offers us, like whether or not life exists elsewhere in the Universe.
No alien that we know of has ever come to Earth.
As far as not seeing aliens, some aliens are microbes. Probably most are microbes. It’s hard to see microbes without a microscope.
I don’t want to move to Mars. It’s a very dangerous environment, you’d be indoors all the time or else wearing a spacesuit for short adventures outdoors. I don’t even think it’s advantageous for humans to go to Mars at all when we could just as well, and for much less expense, send our probes and robot rovers there instead. There is no good scientific reason to go to Mars.
People will go, however, because not everyone is scientifically minded.