Some comes down to power hierarchy too. Presenting up, I'm much more likely to try to appear professional. Presenting down, I go for approachable.
[edit] maybe you could make this argument for a language like Chinese. At least for characters with semantically meaningful radicals.
Err, actually they do. If not the line itself, the font's overall design does. A font can look casual, another official, fun, etc.
In fact this very post uses the "Humor Sans" font for the graphs for exactly this very reason. To make labels appear more casual.
It helps because it focuses attention on higher level elements over nitpicking on details
https://balsamiq.com/?gclid=EAIaIQobChMI5cTZhKzm4AIVtx-tBh3O...
For xkcd plots, it is clear that someone put an extra effort in making it look sketchy.