Summary Cards can be used for various types of web content including blog posts, news articles, products, etc. It gives the user a preview of the content before they click through to the website.
Twitter has now updated the cards so that people using the Twitter app for iOS and Android will start seeing a little bit more information about the content people are sharing with them through links.
We've updated Summary Cards! iOS & Android users will start seeing more detail about web content in their timelines. pic.twitter.com/gZv2B6vham
— Twitter (@twitter) July 14, 2015
As others have pointed out, the update essentially makes the link sharing experience a little more like Facebook and other social networks.
Anything that can help your content stand out in people's Twitter timelines should help in the traffic referral and engagement departments. I don't see any reason why anyone producing content shouldn't take advantage of this. It's just a matter of inserting some simple meta tags into your website's HTML.
Keep in mind that Google is utilizing more and more tweets in its search results too, and tweets with more engagement are probably more likely to show up.
Twitter recently deprecated its Product Cards, and instead points product links to Summary Cards.
Images via Twitter