You have to realize that artists were already using Twitch to do what they are now providing a category for those same people to stream under. I've watched clay, 3D, 2D, etc artists on Twitch; I've watched/listened to DJs, including deadmau5, play with their equipment for hours on stream. These same people also tend to play games now and again, and a lot of the graphical artists tend to have gaming themes. The same is probably true for all of the streamers that will use this category.