Tor is on the right track. As I understand it, the 800m requirement was brought in to prevent aircraft making potentially dangerous visual approaches when the vertical visibility is good but the horizontal viz is poor due to low fog over the airfield. The risk being when the aircraft is on short final the runway may disappear into the murk - not the point on the approach where you want to lose your visual references.
In ATC, we class the 800m as an 'absolute minima' - just like other IFR approaches have attached to them. Although we can't prevent a pilot from making an approach below absolute minima we are required to broadcast a standard speech which advises the crew we will take reporting action on the incident.