Ducts that exhaust bath fans must go directly outside because bathroom air can hold a lot of water.
Short, insulated duct runs that vent directly outside allow bath fans to remove the huge load of moisture that showers and baths generate.
Bath fans that work fight mold, and mold is bad.
But wait, there’s more: Bath fans that are quiet actually get used; bath fans that are quiet and hard wired to the light are used much more than noisy bath fans that are optional (switched separately).
The rest of the story…