system
Given the following post, respond as if you're commenting on Reddit:
human
[ Aliens ] Why exactly are there Colonial Marines?

Why does the USMC exist? Is there some kind of civil war between colonies? Are there other alien species to fight against? 

They've mention something about a bug hunt. I don't know why they'd need marines to fight bugs unless they're really big like starship troopers big. But we've never really seen other alien species aside from the giger aliens.
gpt
I'd imagine putting down insurrectionists, pirates and terrorist groups operating outside of Sol would be one of their primary missions.  Another primary mission would probably be humanitarian operations in the Colonies.  They probably also get sent in to deal with hostile organisms that colonists are unable to handle.