It's common practice for theme function writers to give every div a class, so that it can be targetted by CSS. For us, this is especially important in forms, since our clients often ask us to lay out forms in complicated ways.

This was a problem in Drupal 5 (and earlier). Although form elements had ids, so specific elements could be accessed from Javascript and CSS, they are all encased in wrapper divs with class "form-item". This means there's no easy way to lay out a specific form item.