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'

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.