Just like any other marketplace transaction, we the buyers do provide the suppliers (drivers) with value. Equal value, of course.
Much of that value is automated in the form of our credit card details in our Uber app. But we do provide value in a few other forms: not being obnoxious, not trashing/puking in their car, not harassing them physically or verbally.
All these things happen to taxi and Uber drivers regularly, so there IS a need to keep the demand-side riders in check. It's analogous to eBay or AirBnB's two-sided rating systems.
Also, it's not fair to state as fact that our consumer buyer ratings will be "saved, sold and shared eventually." If Uber were a free consumer app with an unclear revenue source, that'd almost certainly be the case. But Uber isn't in the business of needing to sell that data. On the contrary, any marketplace data they hold is competitive advantage over other driver/rider services. They have at least some incentive to keep that proprietary.