A web User control enables you to build a new control from existing controls. By taking advantage of User controls, you can easily extend ASP.NET Framework with your own custom controls.
Imagine, for example, that you need to display the same address form in multiple pages in a web application. The address form consists of several TextBox and Validation controls for entering address information. If you want to avoid declaring all the
Validation controls in multiple pages, you can wrap these controls inside a web User control.
Anytime you discover that you need to ...