Many sites we design are flat and simple with no more interaction or functionality than a Feedback Form - but some are more complicated.
A website can interact with a database displaying information FROM it in the site for the user, and taking information FROM the user and sending it to the database. This functionality requires code written by Developers and different Developers use different methods, so we use several - just to cover our bases!
Developers and Designers are very different creatures and anyone who says they are a specialist in both areas - is lying. We use PHP and ASP Developers that we know we can work WITH and not against, to make sure we compliment each others work.
A large site may well benefit from a Developers work but it is not essential and Development like all complex skills, costs money. So we won't involve it unless you need it.