It makes sense for the Bengals to use the 17th overall pick on Stanford's David DeCastro, the best guard in the draft.
One reason why Cincinnati struggled to run the ball last season was poor blocking by its guards. The Bengals have parted ways with last year's starting guards (Nate Livings and Mike McGlynn) and signed free agents Travelle Wharton and Jacob Bell. Cincinnati, though, would likely find an upgrade over Bell in the draft.
Improving their guard play will help one of the worst inside running games in the NFL. Our friends at ESPN Stats & Information broke down the struggles of Cincinnati when running up the middle:
30th in the NFL in yards per rush (3.3)
24th in yards after contact per rush (1.6)
28th in yards before contact per rush (1.8)
Tied for 21st in touchdown runs (three)