Is there any specific reasons why payday loan companies ask for social security number.
Before lending money, a lender will want to know what the potential borrower's credit history looks like. For typical loans, you might get a better interest rate if you have a strong credit history. For payday loans, you're going to get a terrible interest rate no matter what, but they'll still check for potential major red flags before lending money. It takes something major (like maybe fraud) to get denied a payday loan, but they're still going to check.
Additionally, payday loans are unsecured. That means there's no collateral for the lender to take in the event that the borrower doesn't repay the loan. In order to increase the probability of being paid back, the lender users the borrower's credit as leverage. Failure to pay back the loan will result in a black mark on the borrower's credit report, which will affect their ability to secure loans in the future. This provides negative reinforcement encouraging the borrower to repay the loan.
All lenders need a way to determine if you are creditworthy. They do so by running your social security number through an independent service that provides verified history of your credit. If you do not provide your SS number, they cannot approve your loan request. Lenders identify you through your social security number.