the idea for 650b wheels is simply there because manufacturers know that 29″ wheels are not very practical for the more gravity assisted types of riding, they do however like the way that bigger wheels render all your current forks, wheels, frames, tyres etc useless.
the logical choice was to come up with a wheel size that still renders most of you existing parts useless but is almost impossible to tell apart from a 26″ wheel for most riders and doesn’t have the same physical and engineering limitations as the true wagon wheelers.