I imagine that the routes are plotted by the same method used to get maps and directions on a pc.
I don't have gps but assuming this is correct they are definitely obtuse. Nevertheless, it would be invaluable if you are in a strange city.
I have found that in familiar territory the directions take you through neighborhood routes that may be the shortest distance between two points but take longer by requiring frequent turns and negotiating neighborhood obstacles like speed bumps, stop signs etc. Once I know where I am supposed to wind up I can usually plan a better route using main thoroughfares.