I guess traffic gets a lot cheaper beyond a certain volume. I suspect it's dollars/TB to cents/TB? So they can cut prices for their offerings and have more revenue and growth due to paying less for the traffic. Having these volumes of data also allows them to approach potential clients with some solid data (e.g. we serve 1/5 of image clicks on the net every day and below 20ms response time... I have no data, just speculation)

I could imagine that imgur traffic paid for itself.