what do you understand by "just building a website"? using a Wysiwyg-Editor and create pages offline and load it up? how do you integrate Forum, Blog and other interactive content? DotNetNuke allows you to integrate multiple functions by adding modules. But besides, you can create and edit all content via your browser, create communities etc.

Another option to implement the same functionality would be using ASP.Net technology with provided components from Microsoft and 3rd party companies, however the effort to create a site with about the same functions (including URL rewriting, page management, ...) will be enormeous while you get it in DotNetNuke for free.

It's DNN, your fingers drifted left a little.  And the answer is the same as why you would use any framework, such as ASP.NET, instead of building it yourself.  The answer is that it's already built.  Why would you want to redo something that's already done?  If you have a valid answer to that, then you know why not to use DNN.  If you don't, then you know why you should.

Jeff