IPv6 nodes very often have multiple valid addresses, for example by configuring temporary addresses [RFC8981]. Since IPv6 address space is not a scarce resource, there are scenarios where assigning a complete /64 prefix to an individual host may be advantageous. Two mechanisms for this have been defined in RFC 8273 and RFC 9663.
One scenario where such a solution may be useful is a shared-access network service where a Layer 2 access network (typically Wi-Fi) is shared by multiple visiting subscriber devices. Service providers may have a legal or operational requirement to provide isolation between connected visitor devices, e.g. to control potential abuse of the shared network. Separate prefixes make such isolation much simpler, since there is no need to track multiple individual /128 addresses per host.
This approach has other benefits such as better scaling properties for neighbor caches, etc., which are discussed in RFC 9663. The latter uses standard DHCPv6 Prefix Delegation (DHCPv6-PD) [RFC8415], whereas RFC 8273 uses specially crafted Router Advertisement messages.