There are a couple of reasons to watch how much data your phone is using. First, while unlimited data plans are much more widespread than they used to be, bear in mind that these plans often apply speed restrictions when you go over a certain point every month. And if you're using a limited plan and apps go over your data allowance while you're away from Wi-Fi, it could start costing you money, especially if you're abroad.
To find out how much data your apps have been using on iOS, open up Settings, then tap Cellular. Right at the top, you've got a toggle switch for turning cellular data on or off - turn it off, and apps won't be able to get online at all unless you're connected to Wi-Fi.
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