You get rate limited by cache. The literal physical distance between cache(3) (tiny ram(s) in the processor) and processor can't be zero. So those signals must travel over a distance at the speed of conduction. Having multiple processors allows tasks to be done simultaneously, effectively multiplying processing speed.

But more speed is particularly useful with bad/legacy software(single thread). SolidWorks is a good example.

I think there are two parts to this. There are factors beyond clock rate; clock rate alone doesn't give a full picture. Going from say 166 MHz to 1 GHz brings radical performance improvements without too many drawbacks, once you go above 3-4 GHz, the marginal increase in clock rates becomes increasingly expensive in terms of heat management.

EDIT: Didn't realize there was difference between mHz and MHz.