RoadBikeRides (RBR) generates Google map directions whenever an entire route is created from scratch with the "Auto-Route(follow roads)" check box selected. This is the only way Google can generate route directions. If RBR can generate the directions it will populate the Ride Directions field on the third page of the ride creation process. You can still edit these.
GPX and non-auto route maps cannot use this feature.
There is a case where if you created a map with auto-route, save it, then edit the map, when you go to the description page the Ride Directions will have new Google directions reflecting changes to the map.
In fact, currently, any ride directions will be over-written in edit mode with either the new Google directions or blank. The assumption being some members will assume directions are automatically updated when the map is updates. Unfortunately that wipes out the hard work on directions for other map edit cases. We are the middle of discussing how to deal with this.