How did you choose the provider of your email address?

For Mastodon, you can check https://joinmastodon.org/servers to figure out what server you want to join. But joining one server doesn't mean you can't access people and communities from other servers. It's one big network, but with many different access points (the mastodon servers), each access point has their own niche. These access points each can have their own rules, so be sure to read them before joining.

What the point of joining the fediverse is, I can't answer that for you, that's different for everyone. The only thing I can tell you is that there's a wealth of information available there.