Angle Milling

Angle milling was originally a chebbie thing. Since the combustion chamber is on the outside portion of the head, more can be removed from it be angle milling. You cannot really angle mill a head enough to change the valve angles for any appreciable benefit. The only benefit is being able to remove more from the combustion chamber than with a straight cut. Honestly, there's probably not a whole hell of a lot of benefit to be had at all. Somebody just thought it was cool and tried it and lots of people have copied them since. That's my opinion anyway.