The internal salt levels and acidity of the liquids in our bodies are largely set by our evolutionary history since pH changes how proteins fold.
So if a creature evolved in an extremely acidic environment then later had to adapt to being able to live outside that environment then its internal biochemistry could still end up locked to being very acidic.
The problem is that the Xenomorph blood in the films is very very very acidic. It eats through many layers of metal and even hardened glass. That's strong acid.
But it's implied that Xenomorphs are not natural creatures but rather biological weapons of some kind.
This opens another option: If you're using bioweapons you don't want your enemies to be able to take your weapons apart and analyze them easily. With engineered creatures even a spatter of blood could be used to recover details of how to achieve capabilities you've engineered in.
So how might you protect the details of your creatures biochemistry? well, you could engineer the creatures cells to include tiny hardened Teflon (one of the few substances which can survive it) vacuoles of, say fluoroantimonic acid (pH of −31.3 one of the few materials potent enough to match the capabilities of the alien's blood in the movies) engineered to burst once the cells are detached from the main body.
Detached or damaged tissue now dissolves into unusable gunk, possibly even taking the floor beneath with it.