If you're used to centralised networks like Facebook, you may be wondering why Mastodon and the Fediverse are spread across thousands of sites. Why not just have a single site where people sign up?
There are many important reasons, but maybe the most important is this:
Networks on single sites tend to be bought by bad people when they get popular 😠
It happened to Instagram (bought by Facebook), it happened to Whatsapp (bought by Facebook), it happened to YouTube (bought by Google). It can happen to ANYTHING built entirely around one site.
No matter how much you trust the people who run a site, when someone offers them billions of dollars they may just take the money.
Decentralised networks, where there is no central site, are much more resistant to buyouts.
No one owns the global email network, thousands of indie players like @Tutanota and @protonmail are able to offer alternatives, and if one provider behaves badly you can switch to a different provider.
Just like email, the Fediverse is decentralised, so it is extremely difficult for anyone to ever buy it.
But there are also other reasons why decentralising makes sense for the Fediverse. Maybe the next most important reason is the ability to choose our own rules.
Each site (or "instance") on the Fediverse can decide its own rules for acceptable behaviour. This makes dispute resolution much more civilised, as people who disagree with an instance's rules can move to a different instance (or even start their own).
It also makes a lot more sense than having a massive megacorporation trying to make yes/no decisions on the entire world's disagreements.
Instances can also choose to block other instances. The worse an instance behaves, the more other instances will block it, until eventually the worst-behaved instances are just talking to themselves.
The Masto developers summed this all up well in their official video:
A personal node in the decentralized social network of the future for Adamas Nemesis: blogger, science-fiction writer, artist, and more.