Earlier quoted context omitted.
Well hypothetically say he wanted to go to Iceland, you can't fly from HK to Iceland so you'd have to change in Europe. The obvious route is changing via London Heathrow, this stops him doing that.
Actually the airports of Amsterdam, Frankfurt and Paris are all bigger, better and see more traffic than outdated Heathrow.
If I believe wikipedia, Heathrow is actually Europe's busiest airport (CDG is next).