The Pro has a bigger brighter ProMotion display. It has active cooling, larger speakers, more ports, longer battery life, better graphics performance, better keyboard, and a larger default storage option (starts at 1tb).

If the Base M6 is good enough processor for you but want a more "premium" product, then the base Pro makes sense.

In terms of performance, the difference is active cooling. That will matter to people who are regularly doing sustained CPU or GPU intensive workloads.

I got an M5 Macbook Pro instead of an M5 Air because the Pro has more screen real estate, a much better screen, and its fans means I never hit the thermal maximum of the laptop.

Better display, more ports.

Also, there is no M6 Air yet.