Step-by-step explanation:
Of course, the plumber might not want a haircut, but he might have a wife, a fiancee, or a sister who may need the services of a hairdresser so working for the hairdresser would earn him credit that can be used by any of these people. So in a sense, he is getting things done for others.
Therefore, the time-bank service solves a problem by allowing people to receive services without exchanging money.