I want to ask that y there is need to add user control in web.config & when we add it?? If we want to use user control in different pages,we can just drag & drop it on the form,then y we add it in web.config file.

Do you mean custom controls or normal Web user controls?
The reason to add entry to your web.config is you don't have to register them on every single page you are planning to use them on.

This shows that:
http://weblogs.asp.net/scottgu/archi...eb-config.aspx