One very cool thing about landing pages is that you now have a whole new set of data to examine to determine which keywords are your best.
You may be bidding on the broadmatch term 'widgets'. However you notice in your logs that most people clicking on your ad were actually searching for 'discount widgets'. You can now go back and create another ad group that really targets these new terms.
You will find that not all visitors click through to the merchant. The ones that don't click through probably weren't very interested anyways or they were competitors wanting to see what your landing page looked like.
I'd be curious to hear how overall conversion has been since the advent of landing pages.