Added

In transport and in general in a crowded place, the backpack should be removed and carried in your hand, which you are trying to show. Plus, this is also safety - from the backpack on the back can steal things, money and documents, and you will not notice.

And your invisible backpack is willing to push me in the subway, on the escalator, in the electric car, etc., because you do not notice it behind your back and do not feel its dimensions. Especially when you need it.