T-Mobile has its own, free app for users of its cellular service! The app provides the ability to check how data you've used, pay your phone bill, and even change your plan. Also included is T-Mobile's full support knowledge base and some additional support features.
A great source of information
Everything that T-Mobile offers to its customers, it offers through this app (with a few exceptions). Just as you can change your plan online, upgrading or downgrading, you can do the same with this app. But it's a much faster and smoother experience than using the Internet, and chewing up more data. Speaking of data, the T-Mobile app provides a very, very accurate view of how much data you're actually spending. You can also pay your bill with the app, if you so choose, or get help.
Product and service
Ultimately, this app will only be useful to you if you are a T-Mobile customer already. But you probably knew that. With that in mind, it makes accessing all of the features of your plan much simpler than using a web browser, and it provides far more accurate tracking of data usage than your operating system can, just because the information is from your data provider. Overall, if you're using T-Mobile's cellular service, there's no reason not to be using this app, as well.