What are some uses of a bladder in a fish?

To urinate, if you mean the urinary bladder. If you mean the swim bladder, it is to be able to change its depth in the water. It has a way to vent air from the gills to the swim bladder, and from the swim bladder to the digestive tract. So the fish would inflate the bladder to be able to rise and deflate the bladder to go lower.