After installing the product, pressing the button will trigger a specific push notification in the app. You will be notified that someone pressed the button, and of the tracker's current location.
The button press can thus be used as a safety or emergency button for your loved ones, if you're using the tracker to keep an eye on them.
Pressing the button once can help you check that your GPS Tracker is still alive, as the LED will blink if it's the case.
By performing a long press on the GPS Tracker's button, you will reboot the product, which can be useful in the case it seems not to be working properly.
You can also expand the possibilities of usage for the tracker's button by connecting with IFTTT: the button press will be used as a trigger for automation scenarios.