It Is All About The "Forward Arm"In fact, the question should be "Why not a forward arm?". You don't hit the ball over the center of the plate (as traditional batting tees suggest). You make contact in front of the plate. But there are two other equally important reasons to use a forward arm design:
No More "Dipping"
The forward arm eliminates "dipping" or dropping the hands and trailing shoulder to lift the ball with a "looping" type swing. If you "dip" with the AST, you hit the back of the arm. It forces you to take the bat straight down to the ball, leveling the swing at the point of contact.
Hit The Ball Where It Is Pitched!
The forward arm also pivots and rotates to place the ball on the inside or outside of the strike zone. Then, the arm points in the direction to drive the ball based on pitch location (i.e. pull the inside pitch, go with the outside pitch to the opposite field).
The outside barrier eliminates "casting". It keeps you form swinging "long" and helps you "keep the hands inside the ball". If the bat or arms are extended prematurely the bat head will slap the flexible upright barrier post.
For years coaches have set a tee adjacent to a fence or screen to force hitters to compact their swing. The outside barrier does the same thing except it is a lot more effective. It rotates around the tee to accommodate LH or RH hitters and it moves along with the forward arm to help you keep the hands "tight" when you are working on inside and outside pitch locations. With the outside barrier, you are forced to rotate the hips and torso and extend the hands only at the point of contact. It produces a "quick" bat and more power as well.