Fly is used for four things, in order of importance: 1
Setting up ganks for your half of the map = If an enemy is beyond their half of the map, set up a kill w/ a core
|| 2
Solo Urn deposit = Check minimap, if able: you can pickup urn by timing Fly self-cast once Urn pickup bar is after QUARTER of the bar, not before HALF
|| 3
Solo Objective pushes = If enemy team is pushing objectives and NO ONE is splitpushing, split push w/ Fly so you can get out if needed || 4
Saving a person from ganks/bad plays = If a teammate is getting ganked/doing a bad play vs enemy team (1v6s etc.) Depending on the map pressure, you can fly in and rescue beam someone out.