that would be what the elderly or post-menopausal people who can no longer breed would be useful for though, they have no more need to breed and they can help people who are at the right age to breed do their daily tasks (food gathering, hygiene for the children). This also seems like you're viewing it from a tribal standpoint, as most families nowadays are isolated from other families and it wouldnt matter if there are single adults who could potentially help those families do normal tasks, no one would let a random person take care of their children nowadays.
that is, unless i totally missed the point. I'm pretty tired now though, i just got off my 5 day midnight shift work week so i'm ready for sleeeeeep, understanding stuff is coming hard all of a sudden.
edit: unless you meant WITHIN a family itself, but most adults without offspring won't stick around to help their parents take care of whatever siblings are left, they'll leave to go get married or find a partner to go live with instead