Actually they have multiple apps: FLY, GO4, Go3 (older crafts), Pilot, Ground Station Pro, and a few more.
The FLY app is for newer smaller crafts... originally it was assumed for consumer level, the GO4App was for Mavic, Matrice, Inspire.. a app more designed for Prosumer or Pro usage. Pilot is reserved for the Enterprise Platforms.
The 3rd Party side received DJI SDK, and has developed many unique apps. Some focused to mapping, automated flights (waypoints, POI, etc), some Thermal, Roof Inspection, etc. Several just a different idea of a Flight App that provides features lacking in DJI apps (several of these were later Incorporated in GO4/3 apps).
The FLY App is recently new. It's been developing and gaining features. And now it's apparent will control the
Mavic 3 series... which many disagree with and desire the Go4App.
Why so many apps... development & progression of features & technology.
Once SDK is received by 3rd Party Developers, you'll have multiple options on what app to use for a certain type of flight desired. If using for Map or Photogramatry you might elect MapPilot by MapsMadeEasy. If you want more autonomous missions you might elect Litchi or DroneLink as examples.
One advantage to 3rd Party Apps, they tend to support the full line of DJI Platforms.
Keep in mind, I'm listing about 1/3 of the high quality Flight Apps available for DJI platforms.