Hello karsh,
I agree with ponderworks about saving a baseline. A baseline is how you think the project will unfold according to your plan. However, when you enter the "Actual" hours worked you are able to see the difference between how you thought the project was going vs how it is actually going. This type of information is the variance and it can help you make decisions on how to adjust your plan.
With regards to Actual Hours worked are you wanting to track the percentage complete with a task or the actual hours that a resource worked on the task?
Please be aware that if a Resource works less than the hours they are scheduled Project will add the outstanding time to the end of the task and perhaps extending the duration.
What method of entering actual hours are you using? This will help me to describe other methods available.
Regards,
Michael