For example, they'll often list a phone for say $1000. Then offer you $800 trade in on last years model, and maybe $500 on a two year old model, both objectively much more than the phone is worth. Pair that with EPP discounts that almost everyone qualifies for, RetailMeNot sitewide codes, credit card rewards, and random Samsung offers - you can often upgrade every year if you want for very little money.