The thing is all of the code is used 😅
The only two functions you need to code are at the end of the file: "start" and "update".
'drone' object provides you with very barebones control like setting motor "force", but if you want to fly to some point, you have to program it yourself, same if you want to sequence some "jobs" (e.g. fly to (200, 160) then plant). It's similar in this regard to programming AI for your game: game engine provides you with minimal per-frame control and it's your task to keep track of what AI does to make sure it finishes jobs and not switches them every frame.
The starter code contains this functionality for higher level drone control with job sequencing, but you don't have to use it. You can remove all of the starter code and start from scratch if you want:
function start(drone) { }
function update(drone) { }