They are ephemeral, they hold no data after being closed and backups of them aren't useful. Only the name is needed, therefore a tmpfs is the place to store them.
The name of the socket file/the fact that it exists contains data.
As a result, it's useful to keep them in non-tmpfs paths that can survive a reboot. That way, very simple programs can use them as sort of a config file: `$XDG_HOME/myprogram/do_xyz.sock`.
Additionally, persistent sockets created once at program install time can help coordinate multiple launches of whatever uses them (e.g. by having servers flock(2) the socket or fight over binding to it as a mutex). For programs whose "server" component isn't managed by a service manager, but can instead be launched many times in response to some user action, that can simplify things.
