Why choose a higher RAM frequency than the CPU?

Question

Iron

I assemble a computer from modern iron. Haven't done it on my own for a long time (10 years), a lot of water has flowed under the bridge. I began to study various articles, reviews on relevant sites. It seems that now everything has resulted in a more or less whole picture, from which you can assemble a working computer.
My CPU and Mother:
CPU: Intel i7-7820X (supports max. RAM frequency 2666)
Motherboard: Aorus Gaming 7 X299 (wide range of supported RAM 2133-4400)
But there is one "BUT". As you can see, the components are quite fresh, and reviews on them and trial ready-made configs in magazines and articles began to appear relatively recently. I began to notice that many use in the "top" configs based on this motherboard (or similar), and this (or similar Skylake-X) processor, RAM with a frequency greater than allowed in the official specifications for the processor.
What for? I've read that it doesn't make sense to put in more than the CPU is pulling. Is it true?
And yes, another important factor... now, in the near future, I'm not going to overclock components. But in the future, I want to play around with acceptable factory overclocking, perhaps raise the frequencies of the same RAM, etc.

Answer 1

do not forget that in processes now auto overclocking
and memory are often taken for this
and there are 2 types of overclocking by a divider and
memory is needed on the bus on the bus