there's not really a reason for it anymore besides the server mod and the hosting service
yeah i don't really use the add-ons system myself as a user, but as a modder i find it useful to have a way to push out updates easily and effectively rather than trusting users of them to check back at the forum for updates (a lot of people probably get all their mods through RTB).
hopefully hosting will stay because that's awesome and amazing
i imagine you could probably use the server mod regardless but if RTB is no longer a thing we'd need some kind of equal replacement