In addition to the fine replies pointing at earlier discussion on this subject, and the fact that you can choose how modular you make your code yourself.

One advantage is that it becomes very simple to extend your application via hooks. Allowing things to be done very easily to modify your application.

A great example of that is validating the HTML output of your CGI::Application - something that would be pretty difficult to do if you weren't using the structured C:A framework. (Where you're "forced" to return your text, rather than printing it out piecemeal.)