[FearNotDaniel]

Architects' whimsy doesn't always "make sense" from a practical standpoint but often has some kind of reasoning behind it. In this case it's right there in the article - for purely aesthetic reasons, to invoke the sense of entering an underground "crypt" sort of space when the real treasures (the artworks themselves) lie on a floor above. Makes a little more sense when you know what the article doesn't obviously state, that the whole extension uses a lot of pastiche of ancient Egyptian style, Tutankhamen's tomb and all that, and that the street level entrance is lower than the original main entrance of the National Gallery itself.