any mat serves to isolate, and somewhat formalize the image. The roots of the mat certainly (read-educated guess) come from the longer and more thought out traditions of classic painting.
That said - I'm with Ken. I don't want anyone looking at the mat/frame.
There is a simple optical reason as well - light tones advance, darker tones recede. The mat should not be lighter than the highlights IN the picture (or so goes the academic thinking), but just a few shades off.
There are also other things going on - from the gallery's viewpoint, they want the work on the wall to not "compete" with each other (group show situations), so having a uniform mat and frame style puts every image on a level playing field.
AND - the assumption is that once the image is purchased, the buyer is expected to mat the image as he/she pleases (if they dont like what it came with), as well as frame it in acordance to their particular tastes/setting. Your carefully selected overmat may get discarded, and replaces with any color they wish - after all, they own the thing, who's to tell them what to do with it?