Although sometimes you start out knowing you need a custom control, what more often happens is you find that you're using the same functionality in more than one place, so you start packaging it up into a control in order to remove duplication and improve reusability.  This article takes a look at when and how you should do such things by taking a fairly simple piece of ASP.NET functionality and evolving it from some code on a form to a user control to a fully-functional custom web control.

http://msdn.microsoft.com/asp.net/using/building/webcontrols/default.aspx?pull=/library/en-us/dnaspp/html/aspnet-evolvecustomcontrols.asp