I think this is a recent-ish thing. I didn't have to go through it when creating a company account > 18 months ago.
I'm not sure what problem Apple were trying to fix.
It's probably just a hoop they make you jump through to make sure you're "serious" so that they can cut down on app approval times without hiring a bunch more reviewers.
Definitely seems to be recent. I signed up my company back in July and had to fax in identification forms. No DUNS required.