Let’s use twitter and mastodon for our examples.
Twitter is centralised, while Mastodon is not. This means that it’s possible for Twitter to disappear at any time – the CEO could (theoretically) shut it down and delete anything at a moment’s notice. The company behind twitter has complete control over everything that shows up on Twitter, and can do whatever they please with it.
Even if Mastodon’s creator decided to shut down mastodon.social, all of the other instances would work fine. Mastodon can’t be blocked by the government or your school (individual instances can, but there’s always more). Mastodon can’t be shut down. Mastodon can’t disappear. This is true of all decentralised software, from matrix.org to Pleroma to email. It’s a lot harder for Mastodon or email to disappear than it is for Twitter or Tumblr to do the same.