From what I understand, the purpose is to make the validation easier. IE allows for bad coding styles because it allows you to forget to close tags(<P> seems to be the big culprit, though table tags come a close second) or mix and match the order of the tags and it will still figure out what you mean. Other products like Netscape and Opera are more scrutinous about coding standards.
Good coding with closing tags make it easier to figure out where something starts and stops, thus making it faster to render. Also one less issue to try and allow for cross compatibility for.
I think a big part of it is to force people to go back to the good coding standards of the past. Just MHO though.