What is the part of X11 in X11VNC?

I understand that RFB protocol is used to remote display. X11VNC uses the RFB protocol so that any VNC viewer (RFB-based) can view the display.

Question:

1. Let's assume I have a frame buffer /dev/fb0 for example. I just can write and run the application that reads from framebuffer based on RFB Protocol. In this case, what does X11VNC differs from it.
2. Also, X11VNC itself provides the option to use raw frame buffer. What is the difference between using/not using this option?

x11vnc uses X11 requests to get your screen updates - via Composite/Damage extensions when available or just by doing GetImage requests at time interval and diffing it with local copy. You want to know not only current image of the screen at any point of time but also when it was changed and what area was affected. Also with x11vnc you can track individual window instead of whole screen - there is fair number of x11 features in addition to just rfb server.