What advantages does Fly-Mode have over Walk-Mode? In comparison, fly mode seems really hard to control, and its style of movement is hard to navigate with. Walk mode feels more controllable, though. 

This leads me to the question of does fly mode have its own uses that you can't achieve with walk mode?
There are some key differences that make fly mode better in some situations.


Motions are smoother turning and changing direction are not instant, this may be nicer for recording camera animations.
Speed is handled in a relative manner, so you dont have a fixed speed, it means you can naturally navigate large scenes or a small room in a large scene.
Some controls are different, MMB pans for example.
Walk mode isn't compatible with absolute pointing devices, eg - tablets.
Walk mode uses fixed horizon angle, fly mode allows navigation in all directions with optional z-up alignment.