Originally Posted by Aeroplane
One valid reason I can think of is going between offroad 26" and 29" wheeled bikes. In order to keep your gearing consistent, you figure out the gear inches and keep that the same.
I agree with you for on-road stuff though, not many folks go between 650c, 650B, and 700c at all.
But people do go between different tires often enough. When it snows I put a 32C xcross tire on my rear wheel, which increases the gear inches of my bike.