For me, because it works WAY better than DJI Go. It doesn't crash at all on my system (S7 Edge.) DJI Go would be nice and I'd use it but that crashing.......not fun. I also enjoy the waypoints in Litchi...very easy to program it on the phone or on my desktop or Surface.
I use Litchi primarily for waypoints, but also for a large number of video shots that you can use Litchi's waypoints mode for. I don't think I'm a good enough pilot to get smooth video and transitions, and I can control them better in Litchi.