My choice has nothing to to with rain either....
Hides my choice of fletching.
Protects my fletching in the thick brush.
Reduces the chance of limbs getting in between the shafts and pulling one out.
Adds another dimension to the bow quiver for break up.
That being said, the best design is to NOT have the bottom closed. Much easier to withdraw and arrow, and less noise with the bottom open. Also, wire or zip tie hoops are nice around the perimeter of the bottom. This also insures that you don't have to worry about trash building up in the bottom.
Have fun....