You still need a graphics card to display graphics.
As much as CPU speed is the deciding factor in how well a given emulator runs, certain emulators (especially those that display complex 3D graphics, such as Dreamcast emulators) require specific graphics functions to be supported by the graphics card in order to display any graphics to begin with.
If the video card is old/poor enough to have no support these required functions, then the emulator doesn't work.