Maybe I've used it differently over the years. Some bags, I have one, which are within the carry-on specifications simply don't fit the lockers in the ATRs. They are pretty tight and as someone said not avaliable at all seats. So I have used fastbag as a substitute for trying to squeeze stuff in. Not sure whether I have always had seat+bag, quite likely if an overnight stay expected. Routinely seen them being weighed.