I'd think most GMs, coaches etc. want job security and the only way you get that in the NFL is prolonged success. So you have to continual build your roster/depth etc. The issue is most rookies, no matter where they're drafted, take time to adjust to the NFL the thought is the earlier you're drafted is the more ready or better you'll be. In reality it's not it's more if that player can adjust to the NFL and/or his coaches can use their skill set to get the best of their abilities and even dependent on the team around them. That's why you get guys like Gabe Davis taken in the 4th round can out produce a guy like Henry Ruggs taken in the 1st round by the Raiders. So the reason you keep extra picks is as you never know when may unearth the next great one, see Tom Brady as a 6th Round pick.