We are WELL off-track here, but, my 2p-worth:
1 - old saying - your bags go to the 3-letter code; the aircraft flies to the ICAO (4-letter) code.
2 - it's shorthand - easier to write 'EGLL' than 'Heathrow'. It also makes certain which airport and aircraft is heading to/at/leaving (EGLL vs EGKK rather than just 'London'). A similar site specially for groundies (if there were one) would probably refer to 'LHR' and 'LGW' as that is their shorthand.