Longlats
The key to understanding the difference between visibility and RVR is in the definitions of each. As you correctly state the RVR is measured along the runway ( which is where you will be looking ). If my memory is correct the definition of met visibility is the greatest distance that can be seen over more than half of the sky. So if vis is reported as 800m, then along the runway could be more or less than this value. Visibility is relevant for circling but not so relevant for runway aligned approaches!. Where I work, when RVR is not reported we convert Vis to CMV (converted met visibility ), which is considered almost the same as RVR.
As other posters suggest RVR is more of a check to continue past the approach ban point for a pilot assessment of conditions at the DA.
RVR will always be reported by ATIS ( or TWR ) when being measured during low vis ops. Pilots don't make their own assessment of RVR value.