I think the societal aspect of your collegues being helpful to each other, while not having that exact task assigned, is the reason you could appreciate this setup. I think putting a card on another person's desk and walking away isn't really interrupting, anyway.

Besides, visually seeing stock going low helps the one doing purchases in deciding whether said supply needs immediate restocking, or that there's ample time to collect more low-velocity stock to batch them in one order.

As mentioned elsewhere, the person purchasing may not consume (in this example) milk at all!