As far as I know, Medals aren't really meant for anything but bragging rights. You can get medals from a Medal Master in Pokemon Centers for a variety of things ranging from crossing all the bridges in Unova to adding a certain amount of Pal Pad friends. Supposedly you can "Rank Up" in the Medal HQ located in Castelia.
Here's a list of the Medals you can get:
Neoseeker and Serebii