Lighted medicine cabinet needs GFCI outlet and Dimmer - why?
The GFCI is purely for the outlets (as required by code for outlets near sinks).

The light is separate and controlled by the dimmer. You can also use a normal lightswitch. 

The reason for having both is that you want outlets to be powered even when the light is off. To be able to charge your electric toothbrush while you are not using it for example. Dimming outlets is not a good idea in general.