Microsoft muddied the waters a bit with this word that they aren't even allowed to use anymore. It's variously been used to describe a set of design principles, a UI development toolkit, and a particular subset of Windows software that is designed to run well on tablets and is put together using the aforementioned design principles and framework.
Windows App store apps are Metro-only under one of those three definitions: The framework that we're now supposed to call WinRT.
Which is, of course, not at all the same thing as Windows RT.
HN comments on Microsoft related stories(only negative ones i.e, the positive ones are usually flagged to death even if they rarely get on the front page) sometimes remind me of the blind leading the blind. Some people who don't even use or follow Microsoft technologies are the first to vote up such stories and then comment snarkingly in the comments about things that they seem to have no clue about .
They allow linking to external apps using external payment systems and external installation processes. That's not a great solution for small teams that just want to build apps.
Making a website, creating a setup file, and setting up payment processing and license delivery really isn't much effort - especially when you don't have to pay a 30% commission to the app store.
The benefit of developing for app stores is the extremely smooth process for users to purchase, install, and update apps. Imagine how popular Steam would be if all they had were a bunch of simple games available to purchase, along with links to AAA titles on publisher websites to go buy and download separately.
They allow Desktop Apps as well.