A use case is used to describe interaction between systems and users to achieve a goal. Mostly, a list of possible interaction between the system and the user is identified. The user can be a single person or a group. By using use case, the requirement of a systems is identified from a users point of view.
I had the same question and I got it right. The answer is this because just like how Rudolph's nose and how it stands out from the others. And so if you pick only the important parts and add color then it will stand out.
