Indeed, I think that specific locations are key here. For instance Gatwick is far more practical to millions of people living in Surrey and Sussex, Brighton, Eastbourne, Kent, (despite Eurostar's Ebbsfleet which is located well for many but has poor connections) and Heathrow is good for the whole Thames Valley, Oxford, Reading and more.

Also worth remembering that airports can be destinations in themselves. I have, more than one, been invited to attend meetings at FRA (as opposed to Frankfurt city), CDG (as opposed to Paris itself) etc. That said, as a central Londoner who will typically go to central Paris, Eurostar is usually best for me indeed.