If you have mobile data, you don't want to go over your limit and get charged extra fees. Here's a simple little trick to avoid data overages. In the settings, find the data usage menu and set your plan cycle. This will reset the amount of data used on your next bill date. Then, create a warning threshold to tell you when you are close to your data limit.
For example, if you have 2GB of data, you can set the limit to 1.5GB so you know exactly how much data you have left and can manage its usage for the rest of the month. It's very useful, especially if you're accidentally watching videos on mobile data instead of wifi.