Consider the use cases. The Mac Pro is a desktop, always plugged in and used for the power. The M1 is on a laptop running on a battery where efficiency is more important to improve the battery life.
There's also plenty of time where the system has low intensity use or is idle. The efficiency cores let things like receiving notifications or checking e-mail happen for very little power draw. Fans only need to run during intense use and the system can be quiet the rest of the time.
Who knows, maybe the UI thread only needs access to the graphics card, and the background thread only access to network. It would save a lot of interrupt loops.
This is a bigger deal than it might seem. Fans running means that dust is accumulating in the machine at an accelerated rate, even if you have filters in place. The less the fans have to run, the less dust accumulates, and the longer cooling functions near-peak. A tower that sees sporadic heavy usage especially stand to benefit from fan minimization.
See, for example, this article from yesterday: https://eclecticlight.co/2021/05/17/how-m1-macs-feel-faster-...
Long story short, segregating background work from foreground work means that Spotlight indexing won't cause the UI to become sluggish. I'm guessing it also tends to be more cache-friendly.
At the end of the day you will still need HE Core, they just aren't as important comparing to a Battery devices.
Not to mention Marketing, Die Size cost consideration.
The design challenge of high-performance CPUs today is heat. Too much heat induces throttling. Putting low-priority stuff onto efficiency cores reserves the power budget of the SoC for high-priority tasks.
https://arstechnica.com/gadgets/2021/05/apples-m1-is-a-fast-...