It's because in the past some pilots, having been "Cleared for the ILS" have gone way down below the glidepath to around 1200 ft before finally descending on the GP. At many major airports there is a great deal of traffic below the ILS so, somehow, ATC must protect that traffic and the current phraseology seems to work. Typically, at Heathrow there are helicopters and other SVFR traffic plus IFR inbounds to London City Airport under the ILS fr westerly runways. Operating procedures for this traffic is designed to provide vertical separation at times when it is under the Heathrow ILS traffic.