Because people won't put up with much if there's no benefit for them anyway.
Recently my wife and I were trying to work out why she couldn't submit her address details to a website, even though I could. As we watched her behaviour in filling out the form, we encountered error after error: or rather, exceptional circumstance after exceptional circumstance. And it was clear that very few of the circumstances had been considered, that error handling was the absolute bare minimum, that the form was set up to be almost a trial to use.
I've never been completely happy with this spindly and slightly confusing diagram from the Django Book, ever since it appeared the first edition.
Drupal has a built-in form abstraction system called Form API (or FAPI), which like the Atom protocol consists of two complementary halves. The first is an abstraction of forms to structured data, and is dealt with very well by both online documentation and the excellent Pro Drupal Development. The second is a workflow for turning this data into forms, validating form submissions, and processing valid submissions onto output streams such as the database, the browser or emails to the user.
