When we do squats, we need a good strong base and a stable core. Imagine doing a squat. Would you find yourself more stable with your feet at about shoulder width, planted firmly into the ground or with your feet close together and heels raising in the midst of your decent?
When we attempt a squat, our feet are our roots. We need to keep them flat and planted, almost gripping the ground. If your feet aren't rooted to ground, you're putting yourself at risk. When we push up, we're driving from our heels while the rest of the foot stays planted for stability. You can't do that with your heels in the air! If your feet aren't firmly planted, you're not as stable as you should be. If you're not stable and ascending properly, you're not working your quads the way you should and now you're putting extra pressure on your knees, hips, and back. That's not the hurt we're going for.
What's more, is that this common fault in form also prevents us from working out with our maximum work load. Imagine when we do goblet squats. Squatting with a firm, rooted base starting with your feet could be the difference between using a 17 pound kettle bell and a 25 pound kettle bell. Or a 25 pound kettle bell and a 35 pound kettle bell. Our bodies are the best machines in the fitness industry, but only if we use them properly!
This is actually why I used to train legs barefooted and like shoes like the New Balance Minimus. It allows you to grab ahold of the ground and make the most of your squats.