The insight here is that everybody takes t-shirts as alternative payment method for situations where paying real money would be a social faux-pas. T-shirts are

The insight here is that everybody takes t-shirts as alternative payment method for situations where paying real money would be a social faux-pas. T-shirts are more like letters of gratitude. You can give your friends funny t-shirts to thank them helping you move, but you can't give them $20 instead without insulting them by implying that their time is worth so little.