At the risk of pointing out the obvious, the main reason to create Web API web services is so that they can be consumed. Building a service and hosting it provides no real value unless it’s consumed. When thinking about services, you probably think about using some kind of AJAX to call your service from a web page. However, that’s definitely not the only option to call a service.
When other applications start integrating with your service, they want to call your service from their own code. This is why the exam also focuses on using HttpClient. It’s important to understand how this class works and when you should use it, both in the real world and for the exam.