The interaction between actor and the world is an act that can be seen in everyday life, and we take them as granted and don't think too much about the small detail that needs to happen in order for an act like performing a piece of music to exist. It is similar to the fact that when speaking our mother tongue, we don't think too much about grammar and how to phrase words; they just come out since we are so used to saying them. The actions between an actor and the world, notably, the primary stakeholder (user) and the world in this case, should be thought about in detail, and hence use cases are created to understand how these tiny interactions occur.