Setting r_clamptoedge to “0” removes the fog using ATI video cards, improving a little bit the long range visibility.
/seta r_clamptoedge 0
Values: “0” or “1”
The increased visibility is not much, the area between where the fog begins and ends (which normally is a partially visibile area) is not blocking the view anymore.
It can be a high advantage for some people so this is why most of the servers are restricting it.