Every taxi driver need to be able to make money more than their boundary or they won't money that day at all. They need to roam around the city looking for customer and deliver them to their destination to get paid.
The idea here is to be able to deliver customers to their destination as fast as you can. Gas is not unlimited here, so you need to watch your gas usage and take a trip to the gas station every then and now. If you run out of gas while you are on the road then it would cost you around $150 to continue with the game.