What's the difference between setting your own rate and being offered a rate that you can choose to accept or decline? Is it the marketplace function of Uber (d

What's the difference between setting your own rate and being offered a rate that you can choose to accept or decline? Is it the marketplace function of Uber (determining a price point between riders and drivers) that makes it different? Does being a contractor really require the ability to set a rate that no one is willing to pay?