One of the neatest things about them is that they give you quick & easy access to your most used flies. They can also be used as a fly drying patch. So while I wouldn't store hackled dry flies in them, I would put them on it when i needed a good drying patch.
Of course the wallets snap shut to protect your flies, too. When I'm walking to the next spot on the river, etc I close the wallet & slide it into a pocket. When I get fishing again I open it up & let it hang. If you put your flies in there well they won't fall out.