Battery for RTC
How it's done
If we were to disconnect USB cable from Arduino and plug it in again, date and time that we have set would be lost.
The biggest value of real time clock is to keep track of time even when Arduino is powered off. To be able to do that, TRC has to be equipped with 3V battery CR2032.
We could have added the battery at the beginning, but you would not noticed it's importance.
Insert battery in a slot on RTC module.
Now, you can turn off Arduino and you will see that after turning it back on, RTC keeps date and time set in previous steps.