The primary purpose of the idle process and its threads is to eliminate what would otherwise be a special case in the scheduler.

In Windows 2000 and later the threads in the System Idle Process are also used to implement CPU power saving.

For instance, on x86 processors under Windows 2000, the idle thread will run a loop of halt instructions, which causes the CPU to turn off many internal components until an interrupt request arrives.

On these systems the idle thread will call routines in the Hardware Abstraction Layer to reduce CPU clock speed or to implement other power-saving mechanisms.

There are more detailed sources of such information available through Windows' performance monitoring system (accessible with the perfmon program), which includes more finely grained categorization of CPU usage.