I don't know who the nerd-ass-motherfucker is who first dared to strap a milk crate to his rear rack, but there are legions upon legions of nerd-ass-motherfuckers who owe that guy (or gal) a huge debt of gratitude. It's the poor man's panniers. It's the trunk, or the boot if you're in Old Blighty. It's a place keep your tools or a severed head. Or...um...some milk.