A backpack can be used on a bicycle rack. Put the straps over the seat and post and then bungee cord the backpack to the bike rack. This way commuters and day trippers don’t need to buy a special purpose piece of bike luggage like panniers. Also, once the destination is reached, the backpack can be removed from the bike and then carried conventionally on a human.
Why not just put the pack on your back instead of attaching it to the bicycle? When a person carries a backpack while riding a bike, the weight goes down the arms, to the wrists and handlebars. This is tiring and adds difficulty to steering. On the rack, the weight is supported in stable manner by the rear wheel. When you walk, hike or run with a backpack, the weight is distributed to the hips and then supported by the legs. In bipedal transportation, a backpack uses the strongest bones and muscles in the human body. Similarly, in bicycle transportation let the bike rack, the frame, and the wheel support the backpack instead of your relatively weak arms and delicate wrists.