There is a misunderstanding what assertiveness means.
It is not just about
defense against touchy grabby hands, it is meant to help people in every situation where a violation of their boundaries can happen, i.e. in every social interaction.
Examples could be a relative that criticizes your physique (e.g. a comment of the belly someone developed over the christmas holidays), mansplaining or a repeatedly interrupting colleague. Any sort of interaction bears the risk of violating someones boundaries (that one does not know because how could one if they didn't communicate them [0]). Assertiveness means that someone both knows where their own boundaries are, so that they actually can tell when they are violated and additionally that they have the skills to deal with it.
This can be as easy as challenging the other with a "Oh? Do you think so?", making clear that there is something one does not agree with and that this is a topic the other would not have an easy time to pursue further. Assertiveness is not about making a scene, it is about clearly communicating that there is a topic that will lead to conflict if pursued further.
Basically it is a tool to make people stronger and more confident so that they do not become victims of other people's transgressions as easily.
[0] This does not mean that people shouldn't be somewhat considerate but being overly considerate is nothing more than social self-censorship. Others have to accept your personality as much as you have to accept theirs. It's an interplay.
Edit: This nice line is from Wikipedia: Assertiveness is the quality of being self-assured and confident without being aggressive.