An online coven is a place for like-minded individuals to gather and learn/study together. Here are some examples of why it's good to have a number of covens.
Dragon Magick is a coven dedicated to learning about and working with dragons. Although this coven is not limited to any number of magickal practices, the main point is to work with a dragon companion.
Guild of Mages is a coven dedicated to learning about magick in general and how to work with it. It's a pretty basic coven.
The Path of the Shaman is specifically for shamanistic arts. Although other things are taught about, the main focus is shamanism so anyone wanting to learn about shamans has a place to go.
Spirit Seers is a coven dedicated to working with spirits. This coven teaches how to meet your spirit guides, work magick with the aid of your spirit guides, and how to consult other spirits for other purposes. Spirit Seers also teaches about divination and various forms of kinesis.
You see, each coven has it's own theme and purpose. Different people have different goals and so join different covens to find people who will help them reach those goals and then strive on to reach new goals.
Covens in general should also be like a family. When you join one you're not just joining a learning group, you are joining an online family that you should be as loyal to as you would a family in the real world.