The basic idea of legibility is that the act of making something comprehensible enough to control is itself an act that shapes the thing to be controlled, often with far greater consequences than the control itself. This is because it removes complexity that is deemed as irrelevant that makes it harder to control, and that complexity may be in some way essential to the health of the system.
With conventional placeholders, such as words, we can describe patterns for a large number of situations. On the other hand it is easy to fool yourself (and others) with words, since you can avoid to be specific. Any business meeting can confirm this.
When you draw something you are forced to be specific — and honest.
Our illustration of an “idea” from above is unconventional in the sense that it conveys specific original thoughts of what an idea is. It adds value to the words.
And that is the catch: The drawing must be unconventional to support the conventional words. We have to make sure not to use “words in disguise”. Take a common illustration for “idea” for example, which haunts flip charts all over the world: the lightbulb.
The lightbulb image works on a purely symbolic level, it only replaces the word “idea”. This image of a household item contains no original thought about what an idea is. While symbols like these work well as international replacements for words or icons to indicate a light switch for instance, they convey no nutritional value as illustrations — they are empty.