I used to wear one when I was a trucker. If you drive all day for a living and you're sitting on a stuffed wallet, you're gonna have a backache in no time. With the chain wallet, you just pulled it out of your pocket when you sat down and flopped it on the floorboard. You never had to worry about climbing out of the truck and getting all the way inside a greasy-spoon restaurant or inline at the fuel desk and all of the sudden remember that you left the wallet in the truck!!
BTW, we used to call them "Chain-Drive Wallets" I heard it was because the old time trucks had chain drive instead of a rear differential, so the truckers made a joke out of the guys who had chain wallets.
Also called a Truckers wallet. The chain stopped you from losing it when you were sliding in and out of the seat. A double edged sword for reasons someone already stated. The jacket pocket works except when the jackets home. I've had my wedge stuffed in my pocket so long that the second its gone I feel it. I did think it was cool when I was young of course at the time I didnt have a bike or truck. I never lost it when hanging on the corner or by the bowling alley...