There are plenty of reasons to run Open Sim on your own computers.
If you’re a content creator, you can have as many regions as you want, for free, and load them up when you need to work on them.
If you’re on a school or corporate campus, you can run Open Sim completely behind the firewall, for maximum security.
There’s no lag due to connectivity issues, because the users and the servers are both on the same computer — yours.
And you can work on your builds if you don’t have the Internet connected, like on an airplane.