For free hand honing, you can grind the primary at the same angle that you intend to hone. This allows you to balance the blade on the stone. Essentially, you get two points of contact (top and bottom of the hollow). I'm a guide user, so I attack it differently. I hollow grind at an angle 5 degrees less than I intend to hone. That means that I'm only working the very edge of the iron.
When you use a flat bevel, the secondary can creep up faster, then it's back to reapplying the primary. On thinner irons, like what comes in a Stanley plane, this isn't a big issue. When you have irons that are 3/16"-1/4" thick, you can save a lot of time (not to mention wear on your stones) by grinding a hollow. It will greatly reduce the time and effort required to sharpen.
This is obviously not a full-on dissertation on grinding and sharpening, but I hope it's a little helpful.