Metro style is basically created by Microsoft mainly for user interfaces or UIs. Their key principle is to focus more on the content of the applications, and to rely more on the arrangement and appearance of printed letters on the page and less on the graphics.
Metro style buttons weren’t in the market before the year 2012 because it started together with the arrival of Microsoft Windows 8 and made the whole Windows app more absorbing. Microsoft changed it metro stylebranding to a new facade which looks like tiles in Windows Phones.
Metro style buttons appear to look like tiles with different colors, so as to show an engrossing look and immerse all users to their application. Microsoft has used very modern, fast and immersive to be able to describe its Windows 8 OS.
Metro style buttons and apps are expected to look very similar to each other, and users see almost no difference in look and feel. This change was remarkable because now they are designed to do just well. They look better and can offer so much better UX or user experience. However, no matter how great they look now, their rules are still under development. They might change something here and there, but only to make the user experience so much better.