I've done small things without diagrams, but I almost always make one up just for my own record keeping and so that I don't have to rely on my own bad memory
It also helps me to look at it logically, see the big picture, and see if there's something I'm missing, something I can improve, and where I can add future expandability... I like to future-proof my projects.
On the flip side of things, once everything is wired up, and before closing it all up and covering the wiring, it's nice to take note of, or take pictures of exactly where all the wires are. So in the future if I want to add something, even if it's not electrical, I can be certain that I won't accidentally drill through, or put a screw through some wiring.
