Why Cultivate Chi?

An old friend of mine teaches Tai Chi Chuan. He wrote to me the other day about how his teacher first demonstrated the power of chi to him by sending him flying across the room with a very short and effortless punch to his well-padded chest, reminiscent of Bruce Lee's famous "one inch punch." My friend then went on to describe how he proceeded to cultivate his own chi over years of dedicated practice. The following is my reply to him:
I'm very intrigued by your stories of chi in action, and I'm sure I'd
be even more intrigued by experiencing it directly the way you have.
I confess that I'm skeptical that chi exists as anything beyond
skillful application of physical leverage or biomechanics, although
I'd like to believe that it's more than this. At the risk of asking
the impossible, could you tell me how you define "chi" or what you
understand it to be?

When you write of Dim-Mak, you quite clearly imply that you think chi
is more than grossly physical motion or energy but is, instead or in
addition, a subtler kind of energy that can be harnessed to foster
life and health or to destroy them. But if there truly is such a
thing as a death blow, might it be more simply the skillful
application of sufficient physical force to precisely the right place
in the body to kill someone in a manner that a good forensic
pathologist could explain in strictly physiological or otherwise
conventionally scientific terms? Or is it something more than this?

Furthermore, could you tell me how you or anyone you know (e.g., your
Xi) benefit from cultivating or harnessing chi? Do you use it merely
in defensive or martial applications of tai chi chuan, or to impress
yourselves and others with its power and with the human capacity to
manifest this power, or does it have other uses? Many sages warn
against cultivating so-called siddhis or miraculous powers because
they say that this can divert people from more important spiritual or
overall development. That is, one can become so preoccupied with
figuratively moving mountains that one fails to do what's necessary to
grow wiser, more loving, and more integrated, complete, and fulfilled
as a human being. I guess what I'm asking is how your progressively
learning to harness chi benefits you and others overall. I don't mean
to suggest that I don't believe that it does this for you, for I
suspect that it does. I'm just curious about how you think it does
this.