Its mostly to see if you can catch a tier 5 trophy of each one to hang on the walls.

also a "secret" recipe of a Fishing Hat for a workbench requires 12 different fishes

Variety.

higher grade fish give more raw fish when you cook em, also the anglerfish is used in a recipe for one of the best food items in the game

Originally posted by Sound:

higher grade fish give more raw fish when you cook em, also the anglerfish is used in a recipe for one of the best food items in the game

right, but i thought how much fish u get is more to do the weight?

For decoration as well