the advantages are that you can make the rules and you can tailor the content to what you want to see. This is really important if you don't like drama or posting about certain topics. There can be other advantages as well. The disadvantages are that it is a tremendous amount of work and it is often hard to keep the group active an relevant. It can be nice o just be a group member too. All depends on what you want out of the experience.
The advantages are like running your own business, you decide just how you want it. The disadvantages are like running your own business, it's a lot of work. You need to be dedicated to spending a lot of time, getting new members, keeping it active. It has to be something that's not being done, and lots of other people have to be interested in it.
Basically, what the other said. You can control the forums and the content. Maybe you have a niche idea. Maybe you don't want to be more active in a group than the actual group owner. Maybe a group doesn't exist that fits your needs.