To answer the original question, we unwrap the cables so that we can bake the texture later on. You don’t have to do that step and can skip the cables if you just want to keep Pothead in Blender, but it’s needed in order to export him to a game engine, online viewer, or any other app.
For the Q about the finger, that’s a good example of where you can cut corners if you want! Hopefully ‘ages’ is an exaggeration😅 If it won’t be seen it doesn’t really matter. There have certainly been times where I thought something wouldn’t be seen but it ends up being shown anyway, so I would be careful about that, but overall it’s just for best practice’s sake and not required.
In the bolt screenshot, it looks like the cylinder is extruded straight back from a filled in circle? The selected vertices are showing up as connected to 3 islands, which indicates to me that it’s a non-manifold mesh that would not have a valid unwrap, which would explain why it’s keeping the old UVs and it doesn’t get updated when you unwrap again.
