Napkin sketches are awesome. They show how you really think. Anyone can do fancy renderings given enough time. And while they look great...enh. One of the best designers I ever worked with couldn't draw his way out of a rectangular box.
nothing that would ever be shown to the client
Why? Clients love being including in the collaboration. I get that some design teams believe you can only show clients finished work, and...okay, whatever....but showing this work can help your team avoid a ton of wasted time and also optimize the level of fidelity of submitted work. If a napkin sketch or a callout will do the trick and get the client from point A to point B, no need to increase the fidelity. So show them, and show them without shame!