@rufustheduck Mostly for the local timeline and finding those with similar interests gathered together.

For example, I am on infosec.exchange because I work in computer security (Also known as information security or infosec) so being here means viewing the local timeline mostly shows me posts relevant to my interests, even for those I don't follow.

This lets me more easily find new people to follow or catch news in the field I may have otherwise missed in the floods on a large general server like mastodon.social (which is large enough they may have turned off the local timeline).