Karate has this concept, dachi, which are concrete and discrete standing positions that you learn and are the basis for the kata. At first, a person might think the concept of how to stand would be a pretty unnecessary thing to know in a fighting system. We all know how to stand, we learn from very young. Moreover, fighting is less about standing in one place and more about getting in and out of the way of something. Anyone who ever watched a boxing match, a fencing contest or two people dancing together would assume that the footwork of motion would be more beneficial to fighting than the mechanics of being stationary.
And this first assumption is, to my mind, mostly correct. Moving in combat is essential. Stationary objects tend to get hit pretty easily. Moving out of the way of something, putting the hand up to swat a fly at your ear - these are natural reactions that some karate training tends to deaden within a student. Whereas in an untrained person, an object on a trajectory with their head would instinctively cause back to bend, hands to go up, head to go down, or feet to shift place, undoubtedly some karate training cause people to think about the right thing to do, when they should be doing something straight away.
But the essential question is: Where are you moving to? Are you moving out of the way of something to a position and place that gives you the advantage? Or are you moving out of the path of a punch and into the path of a knee?
Place and position are keys to the concept of stance. The main reason why the way you stand in Karate is of great importance is because of Karate's underlying assumptions. First, karate does not assume a fight. Karate assumes an assault. Karate doesn't assume two people at high noon in a ring with gloves. It assumes one (or more) person(s), coming at you from behind, with a knife. You shouldn't have time or opportunity to dance and feel the other person out. You shouldn't have a chance to jab to get the range. You turn, square yourself with the danger and act. Karate assumes that when you defend yourself it will be on a relatively small area of space, covering a relatively short distance over a relatively short space of time. Karate assumes that one or two shifts of your feet will be all the chance that you get to gain control of a situation. Either you'll shift to good position with good footing, seize a hand, a shirt, a neck, drive a fist into a face, a finger to the eye, a knee to the groin, a foot to shin, an elbow to the sternum or ribs, wrench a wrist, maneuver into a clinch or drive your attacker off balance...or you'll get knifed seven times and left for dead. Your murderer won't be looking at what your feet will be doing.
How you stand (and move to stand) on that small space of real estate as the space between you and your enemy vanishes is the first tool that you have. And if you lose your footing and go to the ground - scared, alone, attacker poised above you, cold earth beneath you, unable to run - Sayonnara!
So we have to make the most of this first tool. That said, certain ways of standing make it easier to do certain things. Widen your legs and your weight lowers, making it more difficult to lift you off the ground. Crossing one leg behind or in front of the other allows you to turn and pivot quickly on the spot. Squeezing your inner thighs together makes it more difficult to strike your groin from below. Placing more weight on one leg than the other makes it easier and faster to kick with the less weighted leg.
So based on the urgency of when the stance will be important, you'd think that karate training would put a great emphasis on getting in and out of stances and less on simply standing in a stance. You'd think this for the simple reason that the transition to a stance - moving from crossing legs to face your opponent to dropping weight to control your opponent - is what makes the stance valuable in the first place. What difference does it make whether you can sit in shiko dachi (low stance) for hours if you can't drop your weight faster than someone can tackle your knee? What difference does it make if your neko ashi dachi (cat stance) looks good in the mirror if you can't close your legs faster than a kick to the groin?
Instead of serving a purpose, the stances are relegated to being dead postures practiced, at best, to strengthen the legs and at worst, because that's what your teacher told you to do. They aren't taught as something that will help you during the worst ninety-four seconds of your life. They are taught as a piece of artifice, something that has an outward appearance, a standard by which you can be judged for your next belt color.
Stance - dachi - is another one of these things in karate that causes us to spend a lot of head time and not enough body time. That is, many karate teachers will tell you how a stance should look in the mirror but not many will ask how it feels to you. Do you feel stable? Do you feel balanced? In what direction is the stance strong? Weak? Could you fall into this position if someone punched at you? If someone grabbed your shirt and tried to stab you? Can you be easily lifted? Can you move quickly from this position? Would it come naturally to you or would you resort to something else? Why?
Form follow function but function does not always follow form. The best criteria of whether your stance is good is not whether someone can push you over or if your knee is over your toe but rather whether the stance gives you advantage over another human being. A stance of a good visual form can work well. But if it doesn't, no matter how good it looks, then you ARE practicing how to stand and lose.