I know one good reason: The fly box floats!! I was by myself fishing the Rio Cochrane in Chile this last November, and dropped a huge C&F streamer box with about 60 nice flies out of my hands and into the current. This was potentially devastating with no way to re-supply them if they were lost, and I was on week one of four. I kept my eye on the box as it floated downstream, and started down river watching it. If floated into an eddy and hung up on a log jam on the opposite shore. It took me a 1/2 mile hike to find a place to cross (nearly lost it as I waded to my chest) and then backtrack back up the other side. But my box was holding in the eddy and I recovered it. For sure a shout of joy.

I have also dropped the C&F boxes out of my cataraft or driftboat, and they are easy to recover do to how well they float.

After each days fishing, I open the box up and let everything dry out. In addition to wet flies if it pouring rain when you are fishing, a fair amount of moisture will get in the box, and you don't want to store the box closed until everything is fully dry.