A 650b wheel has a smaller diameter than the standard 700c (about 1.5 inches smaller), which lets you fit noticeably wider tires within the same frame clearance. On a 650b-only gravel bike, you can typically run high-volume tires in the 47mm+ range, giving you a plush, cushioned ride that absorbs rough terrain beautifully.
This setup really shines for bikepacking and riding chunky, technical terrain where comfort and traction matter more than outright speed. The fatter tires at lower pressures conform to obstacles rather than bouncing over them, reducing fatigue on long days in the saddle.
The trade-off is that you won't have the option to swap to a 700c wheelset for faster gravel races or smoother rides, and 650b tire selection is more limited than 700c. If your riding leans toward rough adventures rather than mixed-surface speed, 650b-only is a solid, purposeful choice — just know you're locked into that smaller wheel standard.