Using bands to help sling you out of the bottom position of a squat is a helpful tool. It's usually accomplished by using a "reverse band" method where you attach the bands to the bar via overhead hooks.
The trouble is, many racks don't have such hooks Even then, it's kind of a hassle to set them up. In either case, this variation can bridge the gap and provide an additional benefit in the process.
Whether you're working with weight that you're not as confident with, or you just need a little assistance in the place where you experience the most compression, the band-assist under the butt can provide you with a tiny bit of help (maybe an assist of 5 or 10 percent) that can make all the difference. It also gives you a depth target so your squats can have some integrity.
Lastly, it can be a real knee saver since the part of the movement with the most assistance – the transitional phase between eccentric and concentric – is also the part where most lifters will feel the most discomfort in the knees.