Use case: DLNA server on portable systems - it makes sense to run these services only when connected to trusted networks like home or friend's one. Other services could use this functionality too, like p2p apps so it would make sense to have this functionality in systemd to avoid code duplication.
This sounds like something that could be implemented easily externally: just define two targets that build on multi-user.target, and pull in the respective services they should pull in, and then somehow isolate to them depending on the location.
This looks like it would be actually useful for servers too, see https://bugzilla.redhat.com/show_bug.cgi?id=837173 where lack of this functionality causes troubles.
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