The usual conceit for old SF movies is that ET wants our resources, or our women, or they are just hungry.

I have no doubt anyone capable of interstellar travel would be able to synthesize anything they needed without having to come here to steal it, and if "their planet was dying", they would have no trouble finding(or terraforming) a perfectly suitable one that was unoccupied

But they are aliens, after all. We have no way of predicting their motives or psychology.

The alien in the film is an inter-species parasite, a highly unlikely critter. Parasites are the most specialized of all organisms, they have co-evolved with their hosts and are constantly in an evolutionary arms race with them. Any creature from "outside" would be unlikely to prey on us, and certainly not prey on multiple species from different worlds.

But the film does make one thing clear: there is no way of knowing what we'll find out there. It would be smart not to give away our location unnecessarily. And if we do go visit them, it would be best not to carry charts aboard that could lead them back to our home world.