The grid helps you to obtain a better/regular placement and routing. For example, if you wouldn't have the grid, you could route at any distance from the next route, allowing you to place the track on the top of the other track. During the routing you can use any grid you want, but take care because the pick & place machines that equip the boards have a certain granularity of the placement, and the PCB manufacturers also have a minimum distance between the tracks. You should check this with your manufacturer.