If someone buys Starbucks gift cards with dirty money, by selling the gift card at a discount, you get laundered money back.
It's could be even more advantageous if you sell the gift cards for bitcoins, which can be sent anywhere in the world and anonymously exchanged for cash in nearly any currency/country through localbitcoins.
Let's say you start out with drug money. You would like to get this money into the banking system so you can use it to buy a house. This is a standard money laundering problem.
If you turn up at the bank with bags full of cash, the bank will ask where you got that money from, and lots of proof that it was legitimate. They will also file paperwork with the government stating that you made suspicious transactions, or they might just deny you entirely. Obviously, you can't say "I got this money selling drugs" so you need a plausible explanation - this is what money launderers do.
So you take your drug money and buy lots of gift cards. Then you resell them at a discount. What you get back is .... cash (or bitcoins which are essentially digital cash). So the bank asks you the same questions. Your explanation is, "I got this money selling gift cards at a loss". This does not hold water, will not be accepted by the bank and thus does not solve your problem, or even take a step towards solving your problem.
A legitimate business being abused by carders is not money laundering either. If that was the case every single online store in existence would be guilty of money laundering. I've heard stories of some businesses that have seen 30-40% fraud rates on card transactions at times.
It's could be even more advantageous if you sell the gift cards for bitcoins, which can be sent anywhere in the world and anonymously exchanged for cash in nearly any currency/country through localbitcoins.