If the simple planar distance and the geodetic distance are significantly different and you use the planar distance for some analysis then the best case will be you got your analysis wrong, the worst case would be that you have bought the wrong quantity of material or built some unworkable or unnecessary. Or you just misplace where poles are and work crews waste time finding them, or you give wrong information for call before you dig queries and somebody goes straight through a pipe or conduit.

GIS systems are an area where software and the physical world can meet in ways many devs don’t appreciate.