The key to this is that most of these companies don't really care whether you have lousy credit or not. They aren't interested in whether you've had financial problems in the past, and they assume that you're having them now or you wouldn't have come to see them. What they look for are three things: are you old enough to sign the contract, have you been on your job the required amount of time, and do you make enough money to repay your loan obligation. That's what they want to know. Sure, they ask for other things too, like banking information and references, but there's usually no credit check. As long as you repay the loan it won't show up negatively on your credit, either. There's nothing derogatory about taking out a payday loan, so it won't make your credit worse provided you aren't late on the payments or have other problems with the repayment of the loan.
Not all companies do it, but if you make enough money and have been on your job long enough, some places will let you borrow up to $1000 against future paychecks. Not everyone qualifies for that much, but if you don't it certainly won't be because of anything on your credit report. You can even apply online, get approved, and have your money transferred into your bank account. You never even have to leave the house! Companies say you'll have the money in 24 to 48 hours, but often you have it less time, and then you can get those bills paid before they end up overdue, take a sick child to see the doctor, get your car fixed, or whatever you need to do that couldn't wait until payday.