Low back seats offer greater stability in whitewater-especially class 4 and above water. It's important they are mounted low on the frame so when high siding is needed, you can quickly scoot over to the tube-either raft or cat. Also, having the seat low means if you get knocked out of your seat, you're not falling down any farther than needed.
Low back seats or no seat at all enables the rower to fully utilize his or her legs, core, and upper body to push and especially, pull back when rowing. High back seats prevent full range of upper body motion to maximize power.
I suppose the tall, high back seats work well for people just learning to read water as it gives them slightly better line of sight. I think they're silly.