When I go to the cafeteria for meals, when I finish eating and return my tray the cleaners often say 吃完了 (chīwánle = "finished eating"). I'm unsure how to reply, and say 对，吃完了 (duì, chīwánle = "correct, finished eating") or some variant.
Question: How should I reply to 吃完了 (chīwánle = "finished eating") after I've finished eating?
I'm unsure of why they're pointing out the fact that I ate my meal (what else would I do with it?).