Most drop bar bikes roll on 700c wheels, and so do most gravel bikes, but many of them are also compatible with smaller 27.5-inch wheels, commonly known in the gravel world as 650b.
With a smaller rim, a 650b wheel allows you to fit wider, higher volume tyres (often measured in inches rather than mm) that see almost the same overall diameter as a 700c wheel.
These hoops allow you to tackle terrain that would put a skinnier 700c tyre out of its depth, without changing the handling characteristics of the bike. Because the tyres have so much more air volume, they are also considerably more comfortable.
That said, 700c setups are usually lighter and faster rolling and are better suited to smoother gravel rides, while 650b wheels are in their element on singletrack and bashing down rocky fire roads. Having the ability to slot in either size makes your bike that much more capable.