I leave my backpack on when re-trying technical sections, for exactly this reason. The only time I take it off is for multiple re-dos on technical uphills where the weight and floppiness are a problem.
I’ve fallen back onto rocks a few times and rolled many times, and I’m sure my CamelBak has saved me from injury.
I used to carry a mobile device in a protective aluminum case within my CamelBak. One fall on my back dented the case. I’m glad that wasn’t my spine. Without the case I think the CamelBak still would have been good enough.
I had a bad fall on the street where I spun about ten times and shredded my hydration pack. I definitely would have been scraped up more without it. A pack is also great for tools, snacks, phone, sunscreen, and a first aid kit.