The purpose of some insects

Well, cockroaches, like rats, have completely evolved and changed to live off of humans in allot of places. Give them a warm house, with dark, damp places inside walls, and they reproduce like crazy. And they are part of a larger cleanup crew in the wild, the rest of which do not move into human homes en mass the way roaches do. You need other beetles, sowbugs, earthworms, etc... To completely clean up everything, which don't adapt to human habitations as well.