Why? Let's face it, there's a lot of noise from a few spammy marketers. If someone has been banned, they can't rejoin unless they get a completely different phone number from their carrier. This prevents spammers from coming in and ruining the community.
Doing this will grant you access to everything in the community.
Download Discord
There is a web version too, but the desktop and Phone apps have more functionality and tend to work better for.