The RVR must be given under a few circumstances, primarily whenever the RVR gives a reportable value, ATC must issue the RVR. There are a few other instances when it is also to be given, depending on prevailing visibility or if there is a difference in reportable values from the touchdown, midpoint and rollout RVR sensors. The RVR value will let the pilot know how far down the runway he can expect to see and also if they are even able to make the approach (depending on the pilot's rating and/or equipment in the aircraft). So really they go hand-in-hand as it will determine whether or not the pilot is able to execute the approach based on the given RVR.