As has been mentioned elsewhere, devoting some cores to background tasks and reserving the others for higher-priority stuff also makes the system feel more responsive.

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.