My question : Why the RVR is so low ? With fog or mist, you won't be able to see the runway.
Not quite. Fog layer can sometimes be less than 100 ft deep with 10+ km visibility above. In such a circumstances, you can see all rwy and approach lights when you descend to your minimum and continue your approach below (M)DA. When your cockpit plunges into fog layer, you loose quite a bit of forward visibility - it's not smart to disregard RVR just because you see the RWY all the time. At worst, you can loose the sight of it in the flare.