Earlier quoted context omitted.
Why?
Usually they scan items faster than you can bag them. The area after the scanner is very small and takes only few items so you have to move fast. I saw below that US Aldi cashiers put the items into a grocery cart. That doesn't happen in Germany as you are expected to do that yourself. Everyone has their own strategy of how to place items onto the line to slow down the cashier at just the right moment.
I am German and and I've never heard of this ^^. You just put the items on the belt in the order that they should go in the bag, then you can pack very quickly.