With the way things are set up right now, a community is centered on a server. This means that if that server goes down, your community disappears. If someone else creates the same community elsewhere, the community is split. If you're on server A and community is on server B but you're defederates then you can't participate at all, even if person on server C would be ok with you participating.

If the community was a Commons then no single server going down could eliminate the community. Individual servers could have mods for their portion of the community so each local community would have control over what they see and if the local community trusts other local community mods then that could be outsourced. Regardless, servers can get what they want to see -- gaming at exploding-heads and gaming at hexbear could coexist in a superimposed state, and servers that are kosher with both extremes would see everything, servers that are kosher with one extreme or the other would see what they're comfortable with, and servers not comfortable with either extreme could see nothing from either while still having a unified community regardless.

This sort of decentralization is why the broader fediverse works well. Lemmy as it currently operates isn't really decentralized since the majority of the power is aggregated into a limited number of servers who have the popular communities.