can u explain with one small examples?
Think like that. You have a very big model (big volume) with small details. It is very hard to see every thing with just zoom in and zoom out. And also rotation is very hard. If you are in fly mode, you can search (navigate) enviroment easly with acceleration movements. And also you can pass though walls and solids easly.
Like Ozgur said, Fly lets you navigate inside your design. Besides large assemblies, I've found it useful for architectural projects to view and move around inside a building. It's almost like Virtual Reality, without the headset.