Dual wheel compatibility lets you essentially have two bikes in one. Run 700c wheels with 35–40mm tires for fast gravel days and racing, then swap to 650b wheels with 47mm+ tires for bikepacking or rough terrain. The smaller 650b diameter lets you fit much wider tires within the same frame clearance, giving you a cushier, more confident ride on chunky surfaces.
The catch is you'll need to buy that second wheelset, which isn't cheap. Also, make sure both wheelsets use the same brake rotor style so swapping is truly plug-and-play, not a workshop hassle.
If you mostly ride similar terrain, a single wheel size is perfectly fine—700c has the broadest tire availability. But if your riding spans fast gravel races one weekend and loaded bikepacking adventures the next, dual compatibility is a game-changer that keeps one bike doing it all well.