Originally Posted by canopus

26" wheels and tires are used and are more available outside of the U.S. So if your touring in other countries you have a better chance of getting parts (tires, wheels, tubes) than if your running 700c. Also on smaller frames the 650b's allow the frame builder/manufacturer to maintain the geometry of the bike instead of making compromises to fit the 700c wheels. 650b's allow better/easier clearance if you are going to run fenders or larger tires. It "typically" isn't used for road racing.