Consider that the typical mid stakes NL player's biggest leak is hyperaggression, and you can see that checking moderately strong and even very strong hands upon occasion has the potential to earn more than betting your hand.

Another thing to consider is that most players aren't imaginative enough to think beyond a deterministic strategy. When you mix it up by occasionally checking showdown quality hands on the flop, you make yourself very difficult to read to the hopeful floater or small pair looking to push you off overcards.