I got a set of 650b wheels for my bike because it lets me squeeze in a bit wider tires. I think for smaller frames, they have some other advantages as well.
Assuming your frame can fit anything, if it is built around/optimized for, say 28mm x 700C, you can maintain the same handling (and same outer tire diameter) with 48mm x 650b, so it does make sense. If you put very wide tires on 700C rims, you will change the handling properties of the bike significantly.
As for the "fad", presumably because mountain bikes are now being sold primarily with 29" wheels (700C), the Big Gravel(™) conspiracy wants to follow that lead.