Re: Both, and a lot more.
TomD
It reduces the tendency to dive if you hit it with the handle angle. I used my 1/2" mortising chisel for most of this kind of stuff, and it has a steep and long bevel, one still gets some dive. I manned up and got the 3/4 and 1", but barely ever use them. When I went to Stanley's his Japanese mater used a wide chisel with a low angle blade, not unlike the blades in the machines. That kind of blade would dive even less.
In fairness to people trying to steer to the angle, the Japanese plane is not cut to the final ramp angle, though the pro's looks like a machine did the job, if one in thinking in modern terms. On the other hand, western practice uses a float, which Japanese planes are fit from a chisel.
About the angle thing, as per usual, the Japanese pros don't have explanations for anything, it is just an observation of mine, but it is true whether they think in those terms or not. A similar thing can be found on the double bevel chisels that have the handle in line. Western chisels. I saw some DVD where a guy used double bevel chisel extensively. Forget the subject, but it is used in the same way ish. You hammer the handle angle, and you ride whichever bevel you prefer, it cuts straight without diving. I think it was used for contouring.
Same concept with an axe. You have two bevels. Some hewing axes have double bevels, or one flat side, and there are all kinds of weird handles to allow you to pick your approach. When hewing with a standard axe angle set up you swing in line, but the tool rides the bevel.