A personal floatation device keeps you on the surface of the water and stops you from drowning, but it’s also SO much more than that, especially on a long expedition.
First, the layers of foam act as an insulator to keep you warm and as body armour if you’re getting bashed against rocks.
It’s your rescue device, equipped with knife and pigtail.
It’s your essential electronics repository. I like to have my phone, my GPS and my compass on short lanyards. Yes, the lines can get tangled but it’s way better than losing the item into the overboard when you’re fumbling with cold fingers.
It’s your survival kit too. A plausible worst case scenario is losing your boat and all your equipment in the rapids, which is why I carry a couple of lighters and a magnesium block to light a fire, a space blanket (not shown), yellow flagging tape to be more visible, and a mosquito net to avoid going insane.
Finally it’s a convenient carrying place for your most accessed items. For me this is typically a small tube of sunscreen, lip balm with an SPF rating, and sometimes a small set of earbuds.