Presumably, there are social reasons for sweet potato washing as well. But, I think the idea here is that the practice had a non-social-cultural purpose at fir

Presumably, there are social reasons for sweet potato washing as well. But, I think the idea here is that the practice had a non-social-cultural purpose at first (washing the hot sweet potato that came out of the fire) but now is widely retained for only a social-cultural purpose (washing fresh sweet potatoes, apples, and other things that are not hot)