jarnold is talking about a situation without debt, that's why you're confused - debt is effectively the difference between present value and future value. Think of debt as the cost of having something sooner rather than later, and you'll start to understand why we need to use a PV factor to price a future cashflow.

Here's a reformulation of the same problem:

If I paid you $200 now, would you pay me $10 a month for 24 months? You have access to a bank which gives you interest of 5% pa compounded yearly.

The answer is no, and the amount that you'd lose out by is $27.94

This doesn't have much to do with the iPhone since we're pricing two different products here, but it's a very important concept to understand in finance.