In the college football world, redshirting isn't used so much to get a physical advantage as it is to get an extra year of eligibility. A new recruit my be redshirted because there's no starting role for him on the team. For example, a team might have an outstanding senior QB, and a solid sophomore ready to step in. So when they recruit a star high school player, they'll offer him a redshirt year so they can promise him two full years as the team's starter, instead of just one.
In the K world, redshirting is often used to give the kid an extra year of cognitive development, with the idea being that they'd be above the average and derive a positive personal image from that.
I knew a little girl who was redshirted because her parents were concerned that she didn't know the alphabet yet. Three weeks after the school year started, she knew the alphabet. And of course, the school was going to cover the alphabet anyway. Silly, isn't it?
