An exemplar is the chosen best way to model and explain a concept.

Why would you use a worse example and how do you know the example you are looking at is the best?

You'd choose something that's less than ideal if you're including multiple examples, because it's unlikely every example is the best one. You might give an example labelled specifically as a bad example. The word exemplar might be more familiar to many English speakers as the form "exemplary".

It can be difficult to know if your example is the exemplar. It can be reasonably assumed that if the creator of the language provides only one example, it at least should be the best one. It is at least the best one in the docs if it's the only one.