Part of the theory, people will tell you, is that if you touch up your edges regularly on a hone, then you can go longer between touch-ups and do all your razors at the same time in order to be more efficient. For example, with a 7-day set, if you like to touch up your edges every 4 shaves, then you can shave for a month before having to get the hone out. Then, once the hone is out, you touch up all 7 edges and you're set for another month.

While this may have been a valid reason at one time, now I think the rotation concept is just a way to rationalize RAD (which we all get).