If I want my gravel bike with 700C wheels to be more off-road capable, it is suggested that I get a supplementary 650b wheel-set. This is the current trend.
If I want my mountain bike to be more off-road capable, it is suggested that I replace my 27.5" (650) wheel-set (and bike, if necessary) with 29" (700C). This is the current trend.
At least the first one makes some sense. The outside diameter of a 650b wheel + wide tire is approximately what a 700C + narrow tire is.