A quarterback can stand out in virtually any sort of team practice. A big arm or a pinpoint pass is evident whether the throws are going to uncovered receivers or through a tight window of the defense. Likewise, a receiver can catch a coach's eye with a leaping catch, and a running back can show off his sharp cuts even when no one is trying to tackle him.
It's not that simple for some of the other players on the field. The Tampa Bay Buccaneers are holding a rookie mini-camp this weekend, and as is the NFL rule for all offseason work, they can't put their players in pads or have them hit each other.