Think of it this way. You build this window in layers. The first layer is the stationary glass and the 3/8"x3/8" rubber, because the stationary glass doesn't fill up the whole opening horizontally. So you need that 3/8" rubber to fill the gap. The next layer will be the channel. Sure, a sliding glass rests in that channel, but something needs to fill that whole circumference of the window opening. There is no other material to do that except for the felt channel. All those layers are held in with the inner window trim.
