Answers to questions are here: https://answers.launchpad.net/kicad/+question/273325
But the point: the CERN guys implemented the push-and-shove router. That required OpenGL acceleration because the default canvas was too slow. Since their effort was initially just getting PNS to work, some of the features of the legacy canvas were not implemented. The developers are now working to implement all of the legacy features into the OpenGL canvas, and when that work is complete, the legacy canvas will go away. There will be only OpenGL.
So rather than getting rid of useful features that are not yet implemented in GAL, the two canvases co-exist and you use the one best suited to your needs. I'm in GAL most of the time.