Allows you to see the camera view, map, heading, telemetry including altitude, speed, distance etc. it also lets you change camera and other important settings. Where you planning to fly with another app other than GO?
Technically, you do not need a phone or the app to fly. But it helps! First and foremost it provides information you need to determine whether or not the drone is ready and safe to fly. Then there is the video feed so you can line up your camera for whatever images you may wish to take. Lastly it provides telemetry; the automatic measurement and wireless transmission of data from the drone. This includes altitude, speed and distance and more.
Before you fly:
1) Read the manual
2) Watch every video you can find on YouTube pertaining to your particular drone. Ignore all the distance tests for now.
3) Read the manual
4) Take at least a dozen test flights on the simulator included in the DJI app
5) Read the manual
6) Read the manual.
Get ready for some scary moments. It's nerve wracking at first but along with practice comes confidence and less stress. Too bad I'm not there yet!