Why would anyone not print out a boarding card knowing that it will cost them money. After this they usually go on to berate FR about the high cost of their failure to print.
Any one of a myriad of reasons.
Broadband down and lack of printer ink being two obvious ones. And before you say don't leave it to the last minute, get real for a minute - it happens. Also, if you're off on a two week trip then you might not be able to do the return boarding pass until the last minute anyway unless you want to have to find an internet cafe in the middle of your holidays.
Personally I still don't get why you need to check in anyway. You've bought the damn flight after all; why can't you just enter all required passport details at that time and print your boarding pass on the spot?