It allows them to confirm what you bought is the same as what you scanned. It also makes sure you didn't miss-count your multiple items. (Either double-scanning an item or failing to scan an item.)
Having now lived in lower-class areas and gentrified areas, stores that can afford a self-checkout kiosk tend to trust poor people less. (At the same time, they over-estimate their losses from theft or accidental shrinkage).