Well, they allow your followers to interact directly with a tweet, by sharing their email address without leaving twitter or filling in a separate form. Neat huh?
The twitter lead generation cards only work with promoted posts (of course) but with such rich media and ease of use you can expect to see lots of these popping up in your twitter feeds. Which of course has the knock on effect that you have to make your own content even more engaging if you are going to grab those fleeting eyeballs!
For more info on the twitter lead generation cards, see the update from twitter here http://advertising.twitter.com/2013/05/Capture-user-interest-with-the-Lead-Generation-Card.html