There are two main reasons for using canting:
1. You have the ability to direct power precisely to the tail and nose, an advantage particularly for rocker boards.
2. You get an anatomically-correct stance so your knees are spared from strains and tweaks.
We made a video that explains it a little better and how it applies to Rome bindings as well:
Rome SDS 2013 Yes, I Cant System - YouTube
Hope that helps.