Mostly just the resilience and control. An outage or censorship incident on one node can be contained, isolated, and users can easily go around it.
"Oh no, my preferred instance went down!" switches to another instance with the exact same content
Also, I think some European governments run Mastodon servers for themselves. Which sounds weird, but makes more sense in an IT security context. Their data, stored on their servers, that they manage. No third party business contractors needed.
Looks sweet and futuristic, but it wouldn't last a week over here in the states. Some dipshit would try crushing it with their lifted diesel pickup to compensate for their tiny pp.
But then again, that can be solved with a pair of concrete bollards. One on each side.