There are plenty of reasons to run OpenSim 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. There’s no lag due to connectivity issues, because the users and the servers are both on the same computer — or, at least, on the same computer network.
With OpenSim hosting prices dropping fast and features and stability improving, it surprises to me that people still ask why anyone should pay for OpenSim. Yes, OpenSim is free. You can go to OpenSimulator.org and download the software and run your own world, at zero cost. But, like much open source software, OpenSim is “free as in puppy, not free as in beer.”
OpenSim does not run out of the box. Rather, it depends on other software to be installed first. This is a common and useful procedure for software development, since most programs depend on preinstalled software or software libraries developed by others. The reason we don’t notice this too much is because the majority of applications comes with the software they depend on packaged into the installation. Second Life viewers, for example, come with several libraries included1 . OpenSim depends on the Microsoft .NET framework, which is only available for Windows users; however, there is an open-source alternative for several other operating systems called Mono. If you are on Windows, you can use both .NET or Mono, if you’re on OSX or Linux, you will have to use Mono.