The Go4 app is what you fly with on your device.
The DJI Assistant 2 is loaded on to your PC so that you can plug the Spark up to it and upgrade the firmware for the Spark.
I think you can change some of the flight parameters of the Spark to tweak it a bit, if you know what you are doing.
I have been flying the Spark since July 2017 and have never once plugged up the Spark to the PC / DJI assistant 2, to upgrade the updates that come along.
I just do the WiFi dance when updating the Spark.
Download the packet via home WiFi, then switch to the Spark WiFi to
install the packet.
So far, so good.