Just my opinion,
If you are a song writer and trying to sell your wares, it would be beneficial to go through an established publishing company. As Gidge stated, they are better equipped to get your material listened to.
If you, however, are interested in performing your own material it might be beneficial to start your own publishing company. They are a link in the middleman chain you can do without.
Incidentally writing your own material means you get wealthy a lot easier. Pete Townsend became a millionaire a lot faster than his "The Who" cohorts.
Taylor