What's the benefit of cap *and* cork? Some beers, like Lindemann's lambics, come in a bottle that resembles a champaigne bottle (but smaller), with a cork -- and a bottlecap.  What does the cap add to this?  Why is it there?

(By "resembles a champaigne bottle" I mean the glass is thicker and the bottom has that "indent" characteristic of bottles whose contents are under higher-than-normal pressure.)
The bottle cap has the same purpose as a wire cage-- to ensure that the cork doesn't pop itself under the bottle's interior pressure.