A lot of users have limited data plan on their mobile phones and often end up paying huge money for additional data usage. So it is always good to have a watch on the data usage on your phone and restrict it when it goes near the monthly limit. If you are a Windows 10 Mobile user, you can also have a look at the data usage on your phone.
To check the data usage on your phone, open the settings menu.
Now navigate to Network and Wireless and click on Data Usage option. Here you can see an overview of the data usage in the last 30 days and the break up usage by the two SIM card (in case of dual SIM phone) and also Wi-Fi. If you click the data usage chart, you will get a detailed usage chart based on apps.
Here you can identify the app which are using the data the most.