is this compulsary to use a UPS with our computer?

Yes, its your wish to have UPS or not. If you just listen music and watch movies then no need of it. If any necessary work like burning CD / DVD or working for any project then You must use it. Even if you work for something secret then you must use UPS with inverter also. If you live where supply of electricity is regular then you may avoid it. Thats all. Have a nice day.

well see i think its depend where you live and what is the condition of power in your area/city/state ! if you are leaving in campus or city where there is no power cut ! its ok then you will not require UPS ! but if you are in area where power cut is often then it will be good to have it ! in order to have long life of your PC ! but as far as india is concerned i dont think there is any city where power cut problem is not happened yet ! correct me if i am wrong !