You go out grocery shopping with someone and spend about $95 on food, the person you are with pick up a few items totaling around $10. The supermarket is running a promo 'spend $100, get $10 off'.
You are paying separately and go through first - again your bill is around $95. The person you are with then buys their stuff, as you are loading your trolley you overhear them whisper to the checkout person 'put the bill together so we get the $10 off' - which the checkout person does. Thus the person then receives the full discount, making their purchases almost free. They never ask you first, or even mention they did this.