Unmentioned in the Wikipedia article is RFC 7368 IPv6 Home Networking Architecture Principles[1] that discusses them as well.

> A home network running IPv6 should deploy ULAs alongside its globally unique prefix(es) to allow stable communication between devices (on different subnets) within the homenet

[1] https://datatracker.ietf.org/doc/html/rfc7368.html#section-2...