In my quite limited experience, the form for high bar squatting is easy to figure out, but people often fail to maintain proper form when lifting heavy weight, and they often don't realize it. I personally have done this several times, where I thought I had good form and then my workout partner would point out that I rounded my lower back at the bottom, or my knees buckled inward, or I used my lower back to lift the weight part of the way up. Unless you are exceptionally mindful of your own body, I think there's a lot of value in lifting weights with a partner who can critique your form.