What it boils down to is that hiding your form and calling requestAutocomplete allows you to reduce the number of steps in the checkout flow by one (instead of taking the user to a page with a form and then showing a dialog on top of that form, just show the dialog and afterwards jump straight to pre-sale confirmation).
If you're talking about the extra autocomplete attributes, it gives site authors a way to use both the standardized autocomplete fields[0] and different name and id attributes for their own purposes.
http://css-tricks.com/snippets/html/autocomplete-off/