Traditionally, craftsmen have rounded their plane irons very slightly or ground the corners round to try to avoid leaving any ‘plane tracks’ in their final smoothing of surfaces. While the use of a cambered iron is desirable, the actual execution of getting the iron cambered can be problematic. Authors have advised their readers to just ‘ put a little extra pressure on the outside of the iron’ during your last couple of strokes of honing, or grind your iron to a ‘very slight’ radius. I can tell you that it takes more than a little extra pressure for a couple of strokes at the end of honing to get a proper camber. Grinding a ‘very slight’ radius is also not really easy . Or repeatable.
Because of the orientation of the frog in a ‘Blum’ plane, and the mounting of the blade on the bottom of that frog, we can grind a very slight camber ( about .002-.003 ) on the bottom of the frog, and a corresponding hollow on the face of the back up iron. The blade ( which is ground/honed straight across ) , is then inserted between them and when the screws are tightened, it is simply bent to the proper camber. This is possible because of the thinness of the blade. You never have to mess with trying to hone in a decent camber. If the blade was ground straight, you will have a perfect camber every time you sharpen.
When to Use:
I think a camber is useful on most planes. Certainly a smooth plane, to eliminate any plane tracks during final smoothing. Jack and fore planes are oftentimes used for final smoothing also, especially with planes that are easily adjustable. They are traditionally used with blades with much more camber, which must then be ground into the blade. Having a cambered frog just helps out even in that regard. Jointer planes can go either way. Some people say you must have a straight blade for jointing. I find that I prefer a cambered blade, because it helps to be able to move the plane to the side to take a little extra off one side or the other during jointing. The camber is very slight, so doesn’t affect the glue joint at all for gluing up.