The same 'conditional' landing clearance exists at Gatwick:
After the departing A300, cleared to land rwy xx...
'By the time the aircraft crosses the threshold, the departing aircraft will be airborne and at least 2000m from the threshold, or if not airborne, will be at least 2500m from the threshold, providing:
By Day only
The controller is satisfied that the pilot of the arriving aircraft will be able to observe the relevant traffic clearly and continuously.
The pilot of the following aircraft is warned
There is no evidence that the breaking action may be adversely affected
The controller is able to assess separation visually'
It's a handy tool to have in high-density single runway ops, but it DOES NOT excuse poor judgement.