Messy writing and drawing indicates that an idea is in-process, and up to discussion. Clean, polished designs indicate an idea is mature, and discourage discussion.

Messiness can be a feature, because it resembles thinking better than clean and structured software-made drawings or text. The latter can often feel stressful, constraining and infuriating to use. (Over)abundance of choices can create a subtle sense that you’re doing it wrong ¹, and make you focus on the style/design/presentation of an idea rather than the substance.

A non-judgmental environment is paramount for the initial stages of a thought. I should be able to express my idea without feeling judged about it being messy, or well thought out, or complete or structured. (Visual Thinking In Action: Visualizations As Used On Whiteboards)

People’s preference for sketches and whiteboards when ideating can stem in part from the “forgiving” nature of these mediums. Traditional scribbling allows capture of information in non-committal form which feels safer compared to software. As We May Ink? Learning from Everyday Analog Pen Use to Improve Digital Ink Experiences

Side note: A thought can be more easily represented in words if it is better developed. Drawing a diagram is often easier than writing, because one can express relationships without searching for correct wording.