rhm

Good to have you along! Thank you for humoring me.

There are a couple of things I see enabled by the suspension forks.

The most important is that they nearly always have a travel of at least two inches for the city/trekking/hybrid versions, and usually 4-5 for a trail hardtail mountain bike. That's on top of what you get from the tire. The second is that they can have damping, and it can be designed for the purpose, not relying on your jiggling corpus or whatever you get from the tires.

The brake stuff is interrelated, obvs. But I think it's an effect and not a cause. You could not have fork blades that flexed two inches and still use a rim brake. I think that's the main reason the MTB side ditched cantis so hard in the mid 90s for V brakes, even before discs got popular. The cable goes right to the caliper and the caliper is on the element that stays with the wheel and no extra stop is needed on the frame in the way of the suspension movement.