Because of stupid people that unstrap from their bindings for whatever reason and let their board go flying down the hill. I saw two loose boards at Snowbowl last weekend, and I stopped another down by the lift lines.
yea, basically they come standard on rental boards, and i believe those are the only people that need to actually use them, becuase of that. a lot of my local slopes have up signs that say all snowboarders need leashes, but nobody ever enforces that rule.
lol, i remember riding up a chairlift, and about 50 feet away, parrallel to my chairlif was another chairlift with a slope beneath it. some kid with a rental board unstraped for some reason and his board went flying into the woods. there was some guy that worked there ridding up the chairlif at the same time, and after he stopped laughing, he told the kid to put his leash on so that wouldn't happen again. only time i have actually seen that rule inforced.
its a great rule and is there for a reason. to keep a board with the new rider at all times, but if you know what you are doing and know your board is gonna stay straped in till you get to the bottom of the slope, then i see no need for it.