For many years mountain bikes all had the same size wheels, 559mm or 26″ to you and me. This worked well and you never had to worry about tyres fitting.
Sometime in the last decade or so, 29″ (622mm) wheels started to become popular, this is the same diameter as a normal 700c road rim, so there was some sort of logic behind the choice. Tyres and rims were a bit scarce to begin with, but these days there is almost as much choice as there is for 26″ wheels. A few manufactures even mixed things up a bit and sold 69ers (26″ rear wheel, 29″ front), and 29ers are now fairly common and most manufacturers have at least one in their line-up.
Then a few years ago along came 650b (584mm). Less than 5% larger than 26″ wheels, all it’ll do is serve to make tyre and rim choice even more confusing, and cause shops to have to stock even more variations of tyres. What can a 650B wheel actually do that a 26″ or 29″ can’t? Can anyone actually explain the reasoning in a logical manner?