I know that they need a back up QB behind Peyton Manning & because of both Peyton's health & age,they need to groom a future QB to eventually replace him. Brock Osweiler gives them that future QB. The Broncos also need a veteran back up QB who can fill in immediately if Peyton was to get hurt this season.
Them trading down had NOTHING to do with Weeden. The players they wanted went early, and they knew the others they wanted were later round picks. They tried to get value by trading down, but failed miserably.
They don't need a veteran QB, they signed Caleb Hanie, who filled in when Jay Cutler went down last year.