First reason, because you have to overlay the mate over the edge the print slightly, therefore it will always be cropping abit of your image. Some people don't care, but other like myself don't crop their photograph.
Second reason, you should never have anything physically touching the surface of the photograph for an extended period of time. Even if it's 100% archival musuem mat board, it will eventually make a mark/blemish or line on the image that cannot be removed, therefore damaging the print.
Also, depending on the hanging enviroment, I've seen prints fade due to sunlight, ect, and where the mat was over the image, the area under the mat was darker.
The actual surface of the print should never be touching anything for a long period of time, as it will eventually cause damage to the surface. Let it be matboard, glass, ect.