It's a convenient metric, since it also can be substituted with a #handbag, #briefcase, and a couple of bags of #groceries on the way home from the office, or a package to be dropped off at the post office for mailing.
Of course, in my case, I also would like to be able to #transport the smaller #suitcase and my #bass #guitar in her gig bag.
Not coincidentally, it's also about the same volume a child will occupy in a vehicle.