While the thumbs-up sign means okay or good in most places, it is an insulting gesture in Greece. With different cultures comes different interpretation of signs, and writers often do not take this into consideration.
When writing fiction, specifically science fiction or fantasy, people tend to make the world exactly like western culture (if they belong to that group anyway). This is most probably not so, and it creates a very good foreign feel if the writer adds culture specific gestures to certain people (for example, people living in isolation on the moon might have formulated gestures tied to the existence on the moon).
Research published by the American Psychological Association suggests that facial expressions are also subject to different interpretations across cultures. A lot of body language depends on culture rather than just human nature.
A lot of gestures and/or expressions come from ancient times and were adopted by the newer ages, even though they aren’t being used for the same purpose. For example, the thumbs-up sign, as far as I know, comes from the Romans, in which Caesar indicated thumbs-up if the gladiator should live and thumbs-down if he should die.
So let’s say the people on our moon used to excavate minerals (until it was banned due to moon instability). It was pretty dark down there and they couldn’t easily get bright light. It was also pretty dangerous due to caves that collapsed, so they would send one guy to check the passages for minerals before they brought the machinery in. This guy, when he saw minerals that needed to be excavated, he would signal his comrades (radio signals didn’t work because of interference by the strange minerals). However, it was dark and dusty, and neither nodding nor thumbs-up could be clearly seen, so he holds his arm in the air for yes and points it down for no. This would eventually seep into society and they might make a smaller version of it (say, a palm up for yes and down for no) and use that instead of nodding (especially if a lot of the population were miners).
So by putting in a small gesture, you can automatically show some history. With it, you let the reader know that he/she is in a foreign place.