The big thing that this will do is allow developers the ability to prototype apps for the console using HTML 5, JavaScript, and CSS. This means that downloadable content and add-ons to games is now possible for developers to create and sell on the Wii U. This is actually a very big deal for the Wii U platform.
Nintendo first talked about the Web Framework for the Wii U back at GDC in March. The eCommerce support offered in the Web Framework is one area that was sorely lacking and left developers without the ability to offer DLC or additional add-ons for their games because it could not be supported till now.
It is unknown if developers will go back and release DLC content for titles already released on the Wii U, but given the number of Wii U consoles that have been sold, we would not could on it. Going forward however, you can expect more developers to use this ability to offer DLC content for the Wii U platform.