Karl Marx said that the world would be divided into people who owned the means of production—the idle rich—and people who worked for them. In fact it is increasingly being divided between people who have money but no time and people who have time but no money. The on-demand economy provides a way for these two groups to trade with each other.
The “transaction cost” of using an outsider to fix something (as opposed to keeping that function within your company) is falling. Rather than controlling fixed resources, on-demand companies are middle-men, arranging connections and overseeing quality. They don’t employ full-time lawyers and accountants with guaranteed pay and benefits. Uber drivers get paid only when they work and are responsible for their own pensions and health care. Risks borne by companies are being pushed back on to individuals—and that has consequences for everybody.
Good piece from The Economist on the implications of the on-demand economy. It's given workers more flexibility at the cost of stability, and an increasingly freelance workforce means delivery of so much of welfare (like health insurance) through employers is more and more inefficient.