I am not sure what it is for but looks very sinilar to the button I have elsewhere on my Evo IX.
Is your car an import and as a result does it have a rear fog ligt added to make it UK MOT legal?
If so you could find it is to work the rear fog light, but that's just a guess as mine is located on the dash not in the drinks holder.
Other than that could be a switch for other electrics such as any additional lighting you may have or ossible linked to stereo ad-ons?
Either way I do not think it is a switch that is on the car from manufacture but will see what other say