I can't speak for Heathrow but in the TMA we occasionally do the exact same thing. There are a multitude of reasons for doing so but the main one is to prevent the aircraft from leveling off at FL80 whilst still keeping it separated from the outbounds at 6000ft. For example, the pressure is 1012mb and you might only have 1 mile horizontal separation form an outbound at 6000ft so you can't descend through it but the inbound aircraft is approaching FL80 and you want to give it continuous descent. Simple, use 7000ft! The aircraft continues descending almost another 1000ft and by the time it approaches 7000, you have horizontal separation with the outbound and can descend further.
It just keeps things moving.