Coaches often give sermons about tightness and posture at the top of the squat after the bar is walked out. Despite their best efforts, many lifters who may be tight and rigid at the top still look terrible going into and coming out of the hole.
While mobility and stability drills have traditionally been used to address this problem, improving a movement requires actual practice of the movement while challenging your limits. Bottom-up squats teach you how to maintain that same rigid position at the bottom of the squat while forcing mobility and stability changes.
If you can learn to create as much tension and stability in this bottom position as at the top while also increasing starting strength, you'll be able to lift much more weight during a conventional squat.