There are a lot of places where it is dificult to build roads. Such as hills, mountains, lakes, rivers, oceans, wetlands, etc. In addition to this roads tends to be built where people live. It is more efficient to build roads between the towns in an area then to build it straight and build lots of smaller roads connecting the towns to the main road. So if you look at a map you will see that the roads tends to go in straight lines connecting all the towns, mountain passes, bridge sites, etc. together. But that means a lot of turns as there are lots of these along the route.