[scriptdevil]

I work in CPU design. It all comes down to saving power given how frequent idle-state entries are (especially C1 enters with almost most wait-for-interrupt operations) Stop-computing isn't well defined. As long as the clock ticks, the frontend will keep fetching instructions. Sure you can keep feeding it NOPs, but instead, we save power by entering idle states.

The quickest to enter and exit (C1) simply clock-gates the core. Caches are preserved. The next c-state might turn off caches too (and thus incurs the penalty of flushing caches on entry and starting with a cold cache on c-state exit). Further C-states might require even more work to enter and exit but consume much lesser power when in that state.

The cpuidle governor decides which C-state to enter since a deep C-state entry and exit may end up consuming even more power than keeping the system running or in C1.