Windows Phone is one of the more carrier-agnostic platforms out there. You can get one or more Windows Phones from every major carrier in the US, and many of the smaller ones like Cricket too. And many of those carriers choose to offer proprietary apps in order to differentiate their Windows Phone devices or build carrier loyalty.
Keeping up with all those apps can be tough, especially if your carrier doesn't make them easy to find. At the same time, maybe you want to use one carrier's apps on another carrier's device. That takes a little trickery. Even then, you still have to find said carrier-specific apps. Loyal Windows Central reader Thals1992 has developed a free app that links to every known carrier app. US Carrier Apps is a simple app that could come in very handy when you're looking for just the right proprietary app.