At it's simplest this tool helps you check out what technologies other websites are built with.
But BuiltWith has a load more to give ... in their words:
Build lists of websites from our database of 46,488+ web technologies and over 673 million websites showing which sites use shopping carts, analytics, hosting and many more. Filter by location, traffic, vertical and more.
Know your prospects platform before you talk to them. Improve your conversions with validated market adoption.
Get advanced technology market share information and country based analytics for all web technologies.