What do you mean by theory behind it? The concept of chi or what?
Because I do know that Tai Chi (and Qi Gong which is the only martial art art I have dabbled in, aside from Goju Ryu Karate which is what my main focus is) is firmly rooted on good body logic. The movements do help develop awareness of your limbs and so on. I still suggest doing some, even if not for use as manipulation.
I am sure that someone better qualified than I can answer this better.
_________________________
'We're all mad here. I'm mad, you're mad." [said the Cat.]
"How do you know I'm mad?" said Alice.
"You must be," said the Cat, "Or you wouldn't have come here."
- Lewis Carroll, Alice's Adventures In Wonderland