sketching

How do you sketch GUIs/Web Interfaces fast?

I've caught myself for a few days now trying to implement a GUI for my ASP.NET MVC application (specifically a contact management section of it), spending huge amounts of time on the code to position and format everything and then I'm not happy with the result. Part of the time goes into the fact that I'm not too proficient/fast with CSS...

What advantages does sketching a user interface on paper have?

Why would I spend so much time on sketching my user interface on paper before designing it in Interface Builder/Visual Studio/Dreamweaver/wxWidgets/etc... ? Can anyone explain me why we should do this? Do you do this? Thanks in advance. ...