Why do the Aliens constantly seep slime out of all their body parts all the time? How could they drink enough liquids on a barren planet to have enough moisture in their bodies to leak all over all the time, and what purpose does it serve?
The resinous slime is used to cocoon hosts, holding them immobile so facehuggers can easily impregnate them. We saw Ripley pulling this goop off Newt in Aliens shortly after dispatching an emerging facehugger.
The Alien's highly acidic body chemistry might explain the excessive resin secretions, as some synthetic resins are produced by chemical processes involving strong acids and organic compounds. A typical byproduct of the neutralization of acid in such reactions is water.
We don't know that the slime is a water-based liquid that would require moisture. It could be something like glycerin, which is slimy, oozy, thick, and slippery and perfect for general yuckiness. There's no water in glycerin.
It probably also serves as some sort of "marking" abilities as well. A dog doesn't necessarily need to pee over everything, and a cat need not leave their hair everywhere, but doing so marks their territory and sends messages to others of their species. Given the xenomorph predatory behavior, it wouldn't surprise me if the slime also works to help them track victims. If you start smelling your type of slime in an area you haven't bedaubed, odds are that potential prey is tracking it around.