Less friction, less temptation to "let it ride for another month and pay it back then" while debt keeps growing, nothing to do or think about, etc.

> How is it different from using a credit card and paying the balance once the paycheck arrives?

You have to pass a credit check to get a credit card.

If, for whatever reason, you have poor credit, you may not have access to a credit card.

A person who needs this service likely isn't getting a decent interest rate (still important to consider even if you plan on paying the balance in full), or a high enough credit limit.