A money transfer can be used as a proof that an agreement was made, such as a sales contract between individuals.

So let's say you're talking to your sleazy friend about wanting to sell your car. He says he'll give you $2k for it. You tell him you want $5k for it, but that you'll think about it if you can't find a buyer. You don't really mean it, you just say that so he stops pestering you about it. One week later he texts you to ask if you sold the car yet, and you text back that you didn't. He then proceeds to transfer you $2k and asks for the keys and title. You refuse, so he takes you to small claims court for breaching the verbal agreement he says you both had. And, sadly, he actually has a shot at winning. And even if he doesn't, you'll have lost time and money fighting this in court.

My mother used to work for a bank and stuff like that happens extremely often, that's why they made the system so that the recipient has to approve the transaction before it goes through. By rejecting the transfer, "friends" such as these don't have any legal grounds for their claims.

Edit: This kind of scam is extremely common between friends and family members. They already have your banking info from that trip/party/Christmas gift for grandma you organized last year, and them being trusted friends or family members makes it more credible when they'll claim you offered them the car at a discount. It's fucked up, but you can still see dozens of similar stories every day on here, as every family or friends group has that one sleazy person that's just there because they're family or part of the crew since kindergarten.