Steam Achievements was first written to trial the c# Monotouch development framework (forerunner to Xamarin) for producing iOS applications. At the time Steam did not have a mobile application for viewing your achievements
so this filled the gap. I had the added benefit of aggregating achievements with your friends. So you could form a team and help each collect any missing achievements.
The iOS app was archived when Monotouch ended and the Windows Phone is now a distant memory.