The Broncos defense played better than where they ranked last season, but they still need help along the defensive line. Brockers is huge (6'6 and 322 pounds) and fills a need along the Broncos defensive line.
SB Nation's Broncos blog, Mile High Report, explains the pick:
This would probably be some kind of best-case scenario for the Broncos. The board has played out that the only defensive tackle off the board is Fletcher Cox. That seems somewhat unlikely to me, but Cox is not the top DT on my board so for the Broncos, this could be ideal and could result in a move down. From what I've been told, the Broncos would like to trade down if they can, so this would be the ideal scenario to do so. However, it'd be very tough to pass on the talent that Michael Brockers possesses. He is my top-ranked DT this year, and I think he's going to be an absolute stud in the NFL. I read that he was given a late-first to early-second round grade by the draft advisory committee, but based on potential alone I think he is a top 15 grade. At 25 overall, this is a steal and Broncos fans would be dancing and jumping for joy.