Why SDL don't directly write video care memory

Dear Sir:
I know that the SDL can directly write the graphics framebuffer,
but my boss let me to explain that" why SDL don’t directly write
the screen to the video card memory? It would not or It can not
on the Linux?". And I want to know the different between common
framebuffer and the graphics framebuffer, The relationship of SDL and X-server,
how do they interact.
thanks you very much.