An extension of natural human curiosity.
Nobody knows. Aliens probably have some liquid, water or not, that they need to survive. Whether they get thirsty for it we don't know because, of course, we are yet to discover extraterrestrial life.
Yes they can!
Aliens need a home and a job.
No. I am not looking, and have never seen convincing evidence they exist.
I know that they found The Reapers around 2180 so they had to find the other aliens before that.
As far as we currently know there are no aliens and therefore at this time they have no importance. However, should we discover that there are aliens then this will be VERY important and may change the world. It is therefore probably quite important to try and find out if there are any aliens.
As far as we have been able to discover, there are no aliens on Mars. It is a lifeless planet (although there is some possibility that there are microorganisms living in the ground).
Sounds like you could be mis-remembering "Invaders from Mars!"
Alien v Preditor
maybe someone will get them and discover that the weird eye monster
No. What makes you think that aliens need doors?
No, aliens do not need a passport to travel to Earth as there is no confirmed evidence of extraterrestrial life visiting our planet.