...Because, as Zizek points out, it's about the importance of lying...

In one of the diplomatic cables released by WikiLeaks Putin and Medvedev are compared to Batman and Robin. It’s a useful analogy: isn’t Julian Assange, WikiLeaks’s organiser, a real-life counterpart to the Joker in Christopher Nolan’s The Dark Knight? In the film, the district attorney, Harvey Dent, an obsessive vigilante who is corrupted and himself commits murders, is killed by Batman. Batman and his friend police commissioner Gordon realise that the city’s morale would suffer if Dent’s murders were made public, so plot to preserve his image by holding Batman responsible for the killings. The film’s take-home message is that lying is necessary to sustain public morale: only a lie can redeem us. No wonder the only figure of truth in the film is the Joker, its supreme villain. He makes it clear that his attacks on Gotham City will stop when Batman takes off his mask and reveals his true identity; to prevent this disclosure and protect Batman, Dent tells the press that he is Batman — another lie. In order to entrap the Joker, Gordon fakes his own death — yet another lie.
  • U.S. National Oceanic and Atmospheric Administration

However, Nolan's next film, Inception, is great because it offers an excellent example of the relationship between time and size. In the movie, the levels of dreaming, from top to bottom, happen at the same time but at different speeds of time—the highest level happening much slower than the deepest level. The same is almost true for animals. A small animal like a mouse has a short life but a fast heartbeat. A whale, on the other hand, has a slow heartbeat and a long life. But the number of heartbeats a mouse has in its lifetime corresponds with the number of heartbeats a whale has in its lifetime. They essentially have the same life but at different scales. This order is like those dreams in Inception, with the highest level approximating a whale (slow) and the lowest a mouse (fast). One other thing: The whale's closest relative is the hippo. One more other thing: the human animal has somehow found itself outside this order. We are nowhere near the size of a whale but we can live longer than one.