She is probably doing it because she likes the work. She feels good earning a bit of money even if she doesn't really need it. The sense of fulfillment is probably worth a lot more than the actual monetary value. Lots of us derive happiness from work or from feeling useful.

I know at least one old timer (early eighties) that gets really happy when heading over to exchange his bag of aluminum cans. He lives in his daughters home and he doesn't really need money. My thinking is that it makes him happy to earn a bit of money completely on his own, at his age.