Its basically just an interest free pay day loan. The history of (and anticipated future) direct deposits is the "collateral" here. And the banks are in a convenient place to "garnish your wages" to cover any outstanding loans and make sure they get paid back first before you spend anymore.

Honestly, it is telling and concerning that "getting your paycheck early" is even a marketing gimmick used by banks at all. If people are living not only paycheck to paycheck, but paycheck to 2 days before paycheck, that is a very dangerous cycle to be in and one that will crash eventually