An anarchist-oriented Mastodon server has seen one of its admins raided by the FBI. The admin in question was working with an unencrypted backup of the Mastodon server, which was also seized.
Most websites track IP addresses, keep them in a database, and use them for various things. You can find a lot of information on this online, however in the case of non-profit social media like Mastodon, it can be useful for tracking bad actors and blocking them from access. Most of the time they are used for commercial purposes.