And there's more !
Just when you thought it was easy - if the aircraft is not fitted with an auto-rollout system (i.e. to keep it on the centreline automatically after touchdown) such as most DC-10s, the CAT IIIc vis. now requires both mid and stop-end points to have 75m RVR as well.
Previously, you only needed to consider the touch-down zone visibility (runways are divided for visibility measuring purposes into 3 zones - touchdown, mid and stop) but now these other factors come into play.
Basil is absolutely right - it's nothing to do with safety, although low-vis/fog operations are complex, especially if you develop a failure of one system or another (or an engine !) at some point in the approach. That's why we go through it again and again and again and bloody again in the simulator - I suspect most companies have similar training regimes to mine, and I do 10 to 15 practices with all sorts of nasty problems in the sim every 6 months, and quite a few for real on-line.
Basil also forgot to add 'allegedly' to his previous post, so I shall add it for you
