Regardless of whether smartphone users are on an unlimited data plan or pay for a limited amount of data each month, they still need to track their data consumption. Go over the data cap on an unlimited plan, and you could end up throttled during times of network congestion. Go over the data cap on a non-unlimited data plan, and there will be an overage penalty to pay. So it generally is a good idea to track how much data your apps are using.
On the latest developer and public beta versions of iOS 12 (developer beta 4 and public beta 3), Apple has made a change to the way it lists mobile apps. Instead of showing them in alphabetical order, they are now shown in order of app usage. The list also reveals the total amount of data each app has consumed. That information can be reset so you can see fresh consumption figures each month. And a toggle next to each app allows you to restrict usage of said app to Wi-Fi connectivity only, saving you from those apps that are taking you over your carrier's data cap. We should point out that Android users have already been able to track this information on their devices.
To get to the stats, you need to go to Settings > Cellular. Scroll down to the heading that reads Cellular Data. Of course, you need to first install one of the two iOS 12 betas. Assuming that Apple does keep this around for the final version of iOS 12, which will be released later this year, this new feature will be able to save you some money, or the ignominy of having your data speed throttled until the start of the next month.