Question

why does fish breath

Answer 1

As we humans breath for living a life, fish also breath for a living.

Answer 2

every human being is made up of cells , one cell or multiple cells, those are really necessary like that every organism needs to respire or breathe to break down the food and produce energy for every single thing.
what is every single thing?
blinking, thinking, running and every single thing.

therefore, we need to breathe and of course fish breathe

they breathe through their gills which are fish's lungs. They breathe the oxygen from the water:)