There definitely are times when you need to communicate something that is more complex than you can easily do with hand signals. I did a dive with my husband and a friend off our boat once, where we couldn't find the wall we were supposed to dive, and we were wandering around on a fairly featureless gravel bottom in current . . . Eventually, I swam up to Peter and wrote in my wet notes, "We're drifting, and HJ can't drive the boat."
That's one of the few times I can think of where I need to say something I couldn't convey with my hands. It's why I carry wet notes on every dive, but it's also why they live in my dry suit pocket, where they don't catch on anything, or dangle, or interfere with my gauges, or otherwise cause problems. They're there for the rare occasion that the pre-dive plan didn't cover contingencies adequately.