although resistance bands are a great tool for people with minimum experience in sports(especially anaerobic) to learn the movement patterns, it also helps advanced lifters target their muscles in a different way and increase their strength. it is also a great tool for people with joint and other connective tissue and skeletal issues and people who are not flexible and so many other occasions. this variation helps you understand the importance of keeping the back flat.
since resistance bands get tougher as you come up during squats, you are going to feel the need to keep the chest up and back flat otherwise, your hip is going to come up while the chest is stable. resistance bands come in handy when you want to do drop-sets. all you need to do is use several thin bands instead of one and do reps to failure, and then free up one band and again do more reps to failure until you are out of bands(while it is a burden to do supersets with barbells since you need to rack the weights back each time with hands).