Here's another over-simplistic view...
Say you want to buy a car and use it to provide UberX service. You finance the car for 5 years at a certain percentage rate. Car payments are $X/mo. You need to make enough money so that you cover...
* Car payment
* Insurance
* Fuel
* Maintenance
* UberX fees
* Your salary
Plus it would also be nice to make a little more money on top of that (called profit). Then it's pretty easy to see that you need to make X trips to/from downtown and the airport in order to make a profit and pay off the car without any financial hardships.