I don’t use systemd-homd myself but I think the main idea was to have portable users and (optionally encrypted) home directories. systemd-homed users are stored in signed json files instead of /etc/passwd and if you move that file to another machine the user can log into the new machine as if they had an account their.

But as I said I don’t have any use for it personally so their might be more to it that I don’t know about