It helps take some of the logic out of the page, making the page easier to
maintain. Also, you can gain some benefits in debugging, intellisense, etc.
when using codebehind or codebeside. All in all, it generally allows for
cleaner code. Also, when used with the Web Application Project type, you
don't have to worry about deploying code within a page since all the
codebehind files are compiled into a single dll for the web application.

Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Expression