In my case, it was because I wanted to put my 26 inch wheel set with 7 spd IGH on my wife's bike, and VO was (is) running a special on a 650b rim with 8 spd IGH for less than the price of the hub alone. The result is great--the larger wheel set fits my frame perfectly (it always looked a bit out of proportion with 26 inch wheels) and my brakes even reach. The change raised the bike about one cm--still plenty of stand over clearance. Going the other direction, from 700c to 650b, could require new brakes with more reach.
CC