Originally Posted by
FrankJ
Adding another strap...I don't see how this is any different/easier/etc. than a backpack.
The messenger design allows you to compress a (large/bulky or compressible) load down close(r) to your back and keeps it from wagging behind you like a puppy dog tail. To stay put it uses your back to bear the load on a bike rather than your shoulders.
The secondary strap can stay put.