It adds animals that you can farm! What's not to like about that? It's a very deep mechanic, allowing you to make all sorts of farms!
Personally I've farmed cows, bunnies, and chickens. It can be very fun.
A small but important note: You can hold a bunny in your inventory! Scoop up a trained bunny and now you are holding a bunny! This is objectively wonderful.
However you should be aware that the animals move slowly, and they will start spawning all over your map at a moderate rate. These animals are not rare, so expect to see a cow, sheep, kitten, bunny, and chicken in every plain that you cross. Maybe you'd want to decrement the spawn rate if you wanted better balance.
Also, please be aware that stock fences don't actually block the animals. You need the special "mob fences" added by mobs redo for that.