We cannot take a straight line between two locations on a map and consider that line the shortest distance between the two locations. With a very short distance —like within a city— it wouldn’t usually matter. But for long-haul flights across continents, then it would matter a lot.
Flat-Earthers are known to use this mistake —whether deliberately or not— as a ‘proof’ that the Earth is flat. A common case they would bring is emergency landings.