Some is personal preference, some have a function.

Shorter vans are easier to get thru shoot thru rests without contact. They are lighter for the speed freaks. They don't catch as much cross wind.And I think Blazers look cool!

Longer vanes steer the arrow better. They are easier to see as the arrow flies. They control larger broadheads better.

Just depends on what you want to get out of them which ones you use.