(Hair) Care

Harpies are more flexible than humans (they have to be!) but it's still difficult to impossible for them to maintain fine motor control while reaching up behind their own heads. "Mutual grooming" is therefore a fundamental building block of harpy socialization, but those that are still new to being harpies may find the very informal and casual attitudes towards personal space and physical contact at the local roost to be a bit much, and seek assistance from prior acquaintances they're more comfortable around.

It's definitely that, and no other reason.