Backing up your horse, besides it's obvious advantages, is great for your horses stomach muscles and for your horses thinking side of his brain. Try it yourself - walk forward - see, you don't even think, just kind of daydream. Now, backup - notice how much more concentrated and aware and focused you are - the same goes for your little darling and his focus on YOU !
Some great horseman say that "the better a horse backs up and/or goes sideways, the better a horse he is".
Control: backup your horse around 3 barrels - see how you do - it's kind of fun!