With MTOS, publishing is done by the author, whereas with many other blog engines, including Wordpress, the actual publishing is done by the reader. In other words, the resulting document is rendered (built) on the fly. While that can cause more of a load on the server, it isn't too much, and has some advantages.
If I make a minor change to a Wordpress template, I don't have to republish anything. The updates will happen automatically. In MTOS, the files have to be rebuilt, which can take some time. I'd prefer to have the computer spend time rendering the content than a human waiting for the process to complete.