why cairo and the pdf imaging model?

[Rogelio M . Serrano Jr .]

Im sorry to ask  a very basic question but the reason behind display
postscript or it derivative eludes me. Is there an old thread somewhere
that i can read?

Im trying to decide whether to use an x derivative running on opengl
or cairo on opengl. it seems that the first choice seems easier than 
the
second. I now have an OS optimised for gnustep with fbdri. Im trying to
decide the next step (no pun intended) to take.