The foundation of keynsianism is the fact that an idle worker's lost productivity is waste, and putting that worker to work generates something of value. Obviously doing useless work is useless, but making a market so that the worker can be connected with a consumer is a net win. Fiscal keynsianism, aka printing money and handing it out, creates that market by giving consumers buying power to buy things they want, motivating the worker to do work creating value instead of sitting idle wasting their labor. It's a catalyst to activate the real economy.