Another reason for joining is network is for hosting. Some network will host your site and give you unlimited space and bandwidth totally free. And unlike a paid web host provider like CommuniTech, they truly mean unlimited. Because their income depends on your traffic. The more traffic you do the more money you both make.
Networks make money by having sites within the network show banner advertising.
The more banners they show, the more everyone makes so they want you to run as much traffic as possible. There are many networks for you to check out. You should select your network carefully. They all have different requirements and revenue sharing plan. Some will host your site, some require you to find your own host.