Many things! Email, web access, video and music streaming, applications, games, video and photo uploads. All of which can be processed in the background if the appropriate settings are in place. If the battery is also draining fast then there may certainly be some background processes causing the trouble. Check what applications are using the most data by going to Settings & Cellular. You can also see what applications are being used in the background by going to Settings, General, Background App Refresh.
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