Nick Szabo has some writings on the economic shift to hourly wages and proof of work; because of the local knowledge problem (nobody can be an expert on everything, lots of information is hidden or inaccessible or tacit, and it's extremely difficult to determine value), looking at the labor or work which went into a product or service can be a simple fast heuristic for estimating the value of something - since if it was not at least that valuable, the actors who produced it and have more local or tacit knowledge would stop producing it.