It's for non-server machines which will be running other things like the user's browser. You won't get the same performance on the shared cores so sometimes it's better to just run on the free ones, and without checking/adjusting it's just easier to use n-2 cores.

In a production environment you do want to eliminate the other processes.

Thanks. What does n-2 cores mean?

n here is the number of cores

So subtract two. Leave two free.

You want to use better localized memory structures in faster cache, and you want to allow the use of the higher frequency/IPC cores instead of being thermally/memory bandwidth limited by all the cores being loaded at lower frequency all the time.