Never - and I strongly advise anyone else to avoid falling prey to them.
Sure, you get money, but the amount you have to pay back with interest/fees is staggering, and most people don't even realize it will come to so much, causing them to forgo paying another bill so they can pay off the loan (and forget about making a payment arrangement - they want it all) , then they fall behind on other bills and the vicious cycle starts again.
You aren't helping people ... you're robbing them.
Payday loans are bad, bad, bad.