With traditioanal, it opens up the sight window a bit better then holding straight up. My point on target with my longbows is about 60 yards, so anything in the 20 yard range, the point of my arrow is around 18-20 inches below the target. If, I don't cant at all, I can not see the target andarrow. It also helps in keeping the arrow from falling off the shelf or rest. Howard Hill used to cant a bit more on close in shots.

When I used to shoot compound, I always had a slight cant. It was just the way I held comfortable. But I used to have to find sights that would allow for this.