Unlike traditional loan products such as a car loan or home loan, that are directly related to the purchase of a specific asset, there are no restrictions in how you use funds from a payday loan. Your online payday loan can be used as you see fit.
The funds are effectively the same as cash. You know best how to manage your finances, what bills need to be paid, what purchases you want to make, and so on.
Your only obligation is to repay the loan* with interest as specified in the loan agreement. What you do with the money is entirely up to you!