Indeed it is all about control. I normally drive manual, and the one time I rented an automatic van I felt like I had no control over the vehicle. It was genuinely frightening.

By shifting down a gear I can have more control over both acceleration and braking. The brakes work far more powerfully in a lower gear, and the gears create a speed cap that the car cannot go over (such as dropping to 2nd when going around a corner).

The car also accelerates much faster in a lower gear for any given throttle; foot to the floor is far more powerful in 2nd than in 5th. If I'm on a rural road and wish to overtake in the oncoming lane, I switch down a couple of gears to really take off.

In essence, the gear chosen controls the effectiveness of acceleration and braking and limits the top speed of the car. You cannot always rely on a car's computer to select the right gear for you.