I think the most important goal for a design sketch, or illustration is to contaminate an idea clearly. I often find myself sketching something after trying to explain something out loud, then realizing its easier to just draw a picture. After the idea is explicitly communicated in a sketch, there can be a more implicit meaning. Is the sketch quickly done with a pen on paper, or a super-clean computer illustration? The way the sketch is done can created secondary meaning behind how fleshed out the idea is, or with a particular audience in mind. Below are a few examples of sketches and illustrations made to convey different ideas to different users.
This was a concept for a low gravity vehicle with articulating legs to be used on a rough surface in low gravity for example on the moon.
Above is a concept bike packing e-MTB with a Fazua drive unit.
This is an illustration made to lead the design of the Danny MacAskill frame project.
This is a link from a Santa Cruz bike frame with the internal grease passages illustrated.
Above is an early concept sketch of a Santa Cruz V1o made using an underlay of components to check frame proportions, as well as graphics ideas.
This sketch shows a concept for a cable routing idea, aluminum plates that screw onto the downtube with a plastic tube routing a cable.