Who is London Heathrow advertising to?

I already expressed, a while ago, my surprise (and a point of skepticism) about finding advertisements for London Heathrow airport in the London underground...and here is another example. Just that this time, a lot bigger! (I know, the ad is a bit dated since it was from before last Christmas)

But what purpose do these ads really serve? has anyone measured what is the ROI on these ads? as a frequent traveler it is usually the airline that I check first when I need to travel somewhere, only when the choice is between two very similar airlines or fares that I factor in the airport...and I doubt an ad in the underground would have much influence in this latter decision...besides that: wasn't LHR's problem that it was, actually, too crowded?