Because that will give others a little bit more information on who you are. Of course, Owning the camera or lenses doesn't mean you automatically become a great photographer. However, the information does help when people read your posts and will have a better idea where you come from. e.g. having experience with a certain camera body or lenses.
So if I'm a member of a medical forum, do I start listing the dimension of my anatomical body parts as my signature? :think:
I'm sure people would be interested to know the size of certain off limit areas.