A case can be made that if the bike world were to start from scratch, 650B would be the NORMAL size. Why not? It’s in between 26-inch and 700C, so it fits a wider range of bikes than either.
True, you can get dinky bikes with 700c wheels, and monster bikes with 26-inchers, but those are mismatches born of either ignorance or convenience. Small adult bikes are good with 26-inch or 650B wheels; mediums should have 650B or (if it's a big medium) 700C, and larges should have 700C. Visually and practically. The bigger the wheel diameter, the better it rolls over bumps, but you should never let bump-rolling trump a good frame design, and you can't fit too-bigga wheel into a smallish frame. It so happens that 650B wheels fit the widest ranges of frame sizes.
In spite of its inherent sense, up until about 2008, 650B anything was weird enough to scare 98 percent of all big manufacturers and bike riders. We were the non-chicken pioneers, and now 650B wheels rule at least the off-road world, and are no longer freaks on pavement.
Still, it’s a common concern: What if I’m touring in Timbuktu and I need a new rim-tire-wheel? We have actually researched this, and there’s no bike shop of any kind in Timbuktu. There’s what amounts to a general store, but it doesn’t sell bike stuff. Surprise: Timbuktu is a hot, sandy place that’s inhospitable to bikes. You go there, you're on your own.
Here are the rim diameters of the three common wheel sizes, so you can see how they stack up:
700C = 622 millimeters = 24.5 inches
650B = 584 millimeters = 23 inches
26-in = 559 millimeters = 22 inches
Actual WHEEL diameters, which includes tires, vary from about 24-inches for a skinny 26-inches, to 29-inches for a 700C x 50mm. And then, for the love of god, there are the PHAT tires that inflate things even more. They're not part of this right now.
A simple but good way to think of them (but not when communication matters, as when buying wheel parts) is large, medium, small.
700C = Large
650B = Medium
26-inch = Small
When you design a small frame around a 700C wheel, you have to do dumb stuff with the top tube, head tube angle, and fork offset to get the wheel out of the way. When you design a big frame around a small (26-inch) wheel, you have to make compensating compromises in either the length of the fork blades or the head tube. That's when 650B makes sense.
It’s not a magic tire size. It rolls over bumps better than a 26, but not as well as a 29 (700C). It turns quicker than the big wheel, less quick than the small wheel. Its proportions are better visually for mid-sized bikes than either.