They don’t need to, it just happens to be in the best interest of Costco, so Costco inserts itself. People buy larger ticket items at Costco because they trust

They don’t need to, it just happens to be in the best interest of Costco, so Costco inserts itself. People buy larger ticket items at Costco because they trust it to work, with a return policy that makes it a safe bet. So Costco needs to sell products that don’t get returned in order to make this strategy work.