Being able to see the runway on approach does not guarantee acceptable minimum visibility closer in, due to slant-angle. Actual visibility often becomes worse once you enter a fog-bank, and the only place where actual RVR can be assessed is on the runway horizontally.
Which is why Converted Meteorological Visibility is such a daft idea. For an approach with low minima (Cat I, II etc.), with a transmissometer right beside the runway, the RVR will usually offer a good guide as to whether there will be sufficient visual reference to complete the approach from DH. But to determine whether a crew should be permitted to start an approach based on whether the tower can see 3500 m or 3700 m from their perch is just bad physics.