They're acting as cache, to hide the latency to your RAM. They're implementing increasingly niche instructions that speed up ever smaller sets of programs. They're mostly sitting idle, because power usage hasn't fallen at nearly the rate of increasing density, so switching them all at the same time is a great way to start a fire.

A significant part of the exponential gains in the '90s and early 2000s was that clock speed kept increasing. That's been stagnant for years because of heat problems.