If your feet aren't in the right place, pointed the right way; not having the weight distributed correctly between them and myriad other things, your martial art simply won't be as effective.
As I was coincidentally deciding to really focus on my foot placement in my own little practice of the taijiquan short form, a post from which I am placing an excerpt below, appeared at Green Leaves Forest, regarding this very topic, specifically in the context of kyudo.
The full post may be read here.
But before we get to the except, some words of advice from The Beach Boys.
A long time ago a teacher once told me to shoot with my feet.
I said OK and tried …
and just by thinking about my feet, my next shot was one of my best ever.
I was shocked and amazed, and so tried to repeat it again and again, but somehow lost it.
A little while later I noticed that when I shot I felt myself riding on the outsides of my feet, which felt weird and went contrary to what many teachers told me is proper use of the feet, which is having them turn slightly outwards like you’re fanning out from your toes and bringing your heels together.
This isn’t an actual large movement you make with your feet during shooting, but the internal feeling.
Just yesterday I was playing around with my empty hands and experimented with old feelings of riding on the outsides of my feet.
On the other hand, when I remember my feet and concentrate on turning my feet outwards like a fan, I feel a proper tension running up the backs of my legs, my butt scrunches up pushing my lower back forwards, stretching my spine, opening my chest, expanding my elbows without effort, allowing me to turn my head easily (kao-muke), and relax my hands.
The success of our tate-sen (vertical line) lies in our feet (ashibumi)!!!