Purpose of partially filled in rear drop out
What you more often see is that the derailer hanger is a separate piece that bolts into what is the filled-in space in your picture.  

But this is for frames that are designed for the axle to fit only that far back.  Generally these are less than top quality frames where having both dropouts match exactly would make it obvious that the frame wasn't quite true (since the wheel would not center with the axle all the way back).

So with one side filled in you are forced to tighten that side, then slide the other side forward or back until the wheel is centered.  They make it look like a "feature" that facilitates alignment.