It's mostly not. Its real purpose is to improve performance on threaded workloads.

Multi-core CPUs work like this: At the max boost a single core might use, say, 50 watts. So if you have 8 cores and wanted to run them all full out, you'd need a 400 watt power budget, which is a little nuts. It's not even worth it. Because you only have to clock them a little lower, say 4GHz instead of 5, to cut the power consumption more than in half, and then you get a TDP of e.g. 100W. Still not nothing but much more reasonable. You can also cut the clock speed even more and get the power consumption all the way down to 15W, but then you're down to 2GHz on threaded workloads and sacrificing quite a bit of multi-thread performance.

So they're not just trying to eek out a couple of percent, even though that's all you get from single thread improvement, because a single core was already near or at its limit. Whereas 8 cores at 4GHz will be legitimately twice as fast as the same cores at 2GHz. But they'll also use more than twice as much power. Which matters in a laptop but not so much in a desktop.

Of course, the thing that works even better is to have 16 cores or more that are clocked a little lower, which improves performance and performance per watt. The performance per watt of the 96-core Threadrippers are astonishingly good -- even though they're 360W. But that also requires more silicon, so those ones are the expensive ones.