If you're running a public Mastodon server but you have additional features not available on vanilla Mastodon, you might want to tell people about them in your server description. (For example some have higher character limits, more poll options, markdown text formatting etc.)
Whatever your server's extras are, they can be a really good "hook" for people who are trying to find a new home on the Fediverse, and it encourages them to read more about your server.