As a really basic example, say you have $1200 and you want to buy something that costs $1200. You can either pay $1200 upfront, or you can pay it in $100 no interest installments over a year. If you choose to pay in installments, you can leave the rest of the money in a high-yield savings account and collect interest on it over that year, rather than handing it over right away. This means you can buy the thing you want without sacrificing the full opportunity cost of the money over the period.

Apple loses the opportunity value of getting that money upfront, but they guarantee you are committed to the purchase regardless. Further, it also allows them to secure future earnings from people who couldn't immediately afford a large purchase, like someone who doesn't have $1200 upfront, but has $100 disposable income a month. That person may have decided to forego a purchase altogether or buy something cheaper if not for the option.