XF 2.2 What's the intended use case for primary user groups?

[PaulB]

What's the purpose of the primary user group field (xf_user.user_group_id)? I've been unclear on this as far back as XF1. It seems like the only two useful options are Unregistered (for guests) and Registered (for everyone else). Is it just a vestigial feature from the early days of XF?

Even the official XenForo documentation recommends against changing the primary user group field: XF1, XF2