A few reasons I can think of.
*The built in one you'd have to hold down while draining the whole cooler.
*The button will get sticky and stuck within the first couple uses without running a lot of near boiling water through it.
*To make it easy to attach tubing to for either recirculating, or draining.
*So you can attach a manifold to the inside without the use of glue or something else.
On Deck: Chav Irish Red
Pipe Dreams: Chai Porter
Primary #1: Empty
Primary #2: Empty
Primary #3: Imperial Oatmeal Stout
Bottle Conditioning/Drinking: Brown Porter, Pale blonde session ale