I can't remember the reason for it - perhaps someone can recall.
What - the reason for the message or the rising and falling in intonation?
Dunno about the latter, but the former...
Heathrow was designed in the era of relatively short span aircraft with a complex inner and outer taxiway system. When larger aircraft arrived, there was an equally complex set of rules about what aircraft could taxy where. There were numerous incidents of aircraft slicing through rudders or other wingtips - I recall going to pick up bits when I was 'Checker' (seems like a lifetime ago now, was only the early eighties!) Hence the message. I look at a chart of Heathrow now and I don't recognise most of it.
TOO