What is the difference between the GET and POST route directions API?

Question

When using the route directions API, the responses that are returned are very different when you do a GET request versus when you do POST request. In the application that I'm building, right now I think I need to do both request types to acheive the desired functionality; specifically I'm trying to display a truck route with a single waypoint visually on the map and textually as directions. The RouteInstructionControl (for the textual directions, https://github.com/Azure-Samples/azure-maps-services-ui/blob/master/docs/RouteInstructionControl.md) only seems to be able to proces GET requests, but there's additional information in the POST request that I also need. Obviously I don't want to have to make the call twice every time but with my current knowledge and understanding I think I have to.

My biggest question is WHY are the responses so different, what is the difference between these two API request types for seemingly the same information and when should I use what? My secondary question is do I really have to use both request types to accomplish my goal?

Maybe if I understood a little more about the differences I could figure out how to simplify here. Any additional information or documentation that anyone can point me to would be very much appreciated! Thank you in advance.