Normally the CPU clock runs continuously and every cycle the program counter increments (or gets changed by a branch instruction of some kind.) If you want to stop the CPU, you have to gate the clock somehow. Maybe a timer that you could configure and enable via software. But that's extra complexity.. and if you use dynamic logic (which is smaller and faster than static logic), you lose state when you halt. Spinning in a tight loop, on the other hand, doesn't require any hardware support.