Why bother asking when reserving at all then? How do people even know how much their bag is going to weigh before they pack? I don't even own a scale. I can es

Why bother asking when reserving at all then? How do people even know how much their bag is going to weigh before they pack? I don't even own a scale. I can estimate my own weight, but not the weight of a bag whose contents have yet to be determined.