"Sometimes you will be in angles where a machine gunner won't be able to hit especially if you are charging or moving forward."
That crouching pose is like some weird anime pose and has no bearing in actual practice. Doing a barrel roll is comedic. The bayonet is also very odd, some of them are on the back foot.
Others have already mentioned it but as a veteran I'll include my thoughts as well. Every military in the world makes you do stupid, sometimes impractical drills or tasks to either instill discipline, instant obedience to orders, or even attention to detail. Troops themselves know the tasks are bullshit but it is a part of any military force, and its not like you can just tell your superiors that you wont do it because you think its stupid.
Yes, I believe the number 1 source of health claims for vets is something back issues, followed by knee issues.
The terrain they operate in offers little to no useful cover, unless you're on the high ground.
Good that NATO troops are not pickup up stupid habits fighting jihadis then, but that underlines that the drills the PLA troopers were practicing are of practical utility because if your unit is ambushed, knowing how to get down quickly with heavy packs without doing yourself an injury is important.
Let me add more context to this. The larger and longer the conflict, the more you need that ruck. Units will be separated. Supply lines will get disrupted and bases will be destroyed. What you carry on your back and hands is the only thing you can guarantee that you will have. So, you practice that reality in all possible situations and when to dump it and when to keep it. And you always prepare for being surprised. When you are surprised, you move with what you have on you.
Arms and armor race coming to a part of the cycle again where mobility is more important than protection. I'm starting to wonder when body armor becomes irrelevant once more, as the reason for it's widespread adoption again was due to most threats being small arms and fragments from enemies who's most heavy weapon will likely be an HMG or RPG here and there, but with a near peer opponent with more access to vehicles and the big weapons and that armor won't protect you from being liquified by HE.
The roll is most likely just practice to get the soldiers used to different movements in the body. You are correct in that it probably will never get used and they would most likely roll to the sides than anything else. The PLA has always been very "light" and focused on mobility if you've seen their training obstacles. The training which we outsiders question does not always show why they do it but they do it for a reason.