Read something about this years ago. In the ?50s?, the FBI studied the 'best' way to carry and contrasted/compared the relative merits of cross-draw, shoulder holster, etc. types of carry. They decided that behind the right hip was the fastest draw with the best concealment, etc. They came up with the FBI draw method, which involved a sort of squatting forward movement and decided the butt of the gun should be tilted X degrees forward (it's not that much) to aid in the faster purchase of a firing grip. It was combat de rigeour of that era. It has been proven to work pretty well - try a vertical holster behind the right hip for a while and you'll find the forward cant is quite helpful.

Caveat: Keep in mind, that there are probably others on here that can give you all the sordid details, and I probably don't even have the ALL of general ideas exactly right, but this was the gist of it, to the best of my memory (which is not what it used to be).