Do you use Safari Extensions? They're add-ons that work within Apple's Safari browser to add functionality. In the past, selecting Safari > Safari Extensions pointed you to a web page where you could browse and select extensions. While that still happens, there's now a link (seen in the screenshot above) that points you to the Mac App Store.
In the Mac App Store is a nicely designed page showing the available extensions. Not all of the extensions that are available are listed on this page; it's my speculation that Apple is working with extension developers to improve the quality and marketability of extensions. While most extensions were free in the past, there are a number of for-purchase extensions in the Mac App Store as well as some that can be tried and then activated with an in-app purchase.
Personally, I like seeing the Safari Extensions in the Mac App Store. Especially if Apple creates a shortcut in the Mac App Store that points right to the extensions page, I think more people will be likely to try and buy extensions.