It is not necessary for healthy fish, but it does help with some fish ailments.
Used as a "preventative" it's merely snake oil.
Some people swear by it, but it's like using elephant repellant spray in New Mexico. Sure you'll never get attacked by an elephant, but is it because of the spray? Or is it because it never would have happened in the first place?
P.S. I'm selling elephant repellant spray, only $29.95 per can.
They asked me to move because I was blocking a fire exit.
If you're flammable and have legs, you are never blocking a fire exit. -Mitch Hedberg