They are in the classroom to make sure the kids learn SOMETHING.

A decent teacher does not need an assistant unless their assistance has been specifically requested. However, who'd allow outside interference when you can select brighter students to lead activities or groups?

All teaching assistants tend to do is spy and nitpick. However, at certain TCs it appears that they have been hand-picked to sleep with the foreign teachers. Probably to ensure loyalty or blackmail material, who knows?