Roads have a purely cosmetic function at this time but can be useful none the less.
You will probably notice your Settlers will ignore roads and walk the most direct path instead, those familiar with the offline games know that they are used to speed movement and flow there, but for now they are just cosmetic.
GriddingWhen planning a new zone for the best layout of buildings and to help maximize the Proximity Bonus it can be very useful to draw a grid of roads and use a Wheat-field or a Well (clicked and hovered but not deployed) to see what areas are build-able and where best to locate your buildings.
I find this most handy for planning Farms and organizing Water, while water can be placed almost anywhere it is easier to keep track of how many wells you use and need by placing them in distinct grids.
As you level up the detail of the roads improve to look more defined as your settlement gets bigger. These changes occur at level 12 and level 20.
|Level 1-11||Level 12-19||Level 20+|