Anyone who has worked with athletes knows that it is not uncommon to see inability to simply bodyweight squat to parallel with proper form. Lack of ankle mobility? Lack of hip mobility? Lack of core stability? Could be one, all, or a combo of two.
What’s the first and easiest fix? Raise the heels.
Raising the heels gives you a little more ankle mobility. It gives you the ability to posteriorly shift and ‘sit back’ when you squat.
PS — Powerlifters and Olympic Weightlifters have been wearing shoes with an elevated heels for years and raising those heels is not bad for your knees.
PPS — If someone can’t perform a bodyweight squat properly, loading them with a barbell isn’t all that smart and you could seriously hurt someone. It’s actually really irresponsible.