This is what I am not able to understand that if CPU is idle for some particular amount of time,then why does the OS tries to schedule more and more tasks to it
Lets use defragging as an example. Over time, your hard disk becomes fragmented and different parts of data are saved in different parts of your hard drive platter. This is not optimal, and it slows down computer performance. So, a long time ago, people developed a process called "defragging", which organizes your hard drive and puts all the bits of data in the same area, so it can be read faster with less work for the drive. This "defragging" process takes lots of time.
So in modern OS's, when your computer detects that you are not using the computer (idle), it automatically begins defragging your disk in the background. This way, whenever you check your drive, it will always be organized. If you check on an older OS (Windows 95, 98 era), you will not see this effect.
There are many tasks like defragging that can be done by your computer while you're not using it. Most of these tasks are organizational in nature and involve sorting and organizing various things on your system to improve performance.
But, again, not all computers have to do this. You can configure windows to automatically hibernate/sleep or even shutdown if you haven't been using it for a long enough time.
Edited by PcPhoenix, 18 January 2015 - 02:10 PM.