Ambulances have many uses but are used primarily the first medical transport responding to a scene of an accident. This term ‘ambulance’ is a universal term across the world. Each state has laws pertaining to the other drivers and pedestrians when they see an ambulance.
An ambulance is large enough to accommodate two patients at a time. It is filled with emergency medical medicine to try and stabilize the patients until they reach the hospitals. Usually there are about two paramedics minimum to each ambulance. 
An ambulance with its lights on racing down the highway means it is rushing to the scene of an accident or there is a patient in the back that is in critical condition. When another driver sees the ambulance with its lights and siren on, by law they must pull over so the ambulance has a clear path. In some instances the city lights will all turn red until the ambulance has made it through its path. If the driver fails to yield to the ambulance, they are subject to fines and persecution by the state law.
Ambulances are also used to transport patients from one medical facility to the next. The ambulance is used because the patient is not stable enough to be discharged and will require 24 hour medical supervision. Some reasons for transferring the patients includes the patient is in need of a special care unit, they are transferring closer to home, or they are in need of a specialty hospital. 
Ambulances are also used to transport not a patient, but a organ. If a person on the donor recipient list finds an organ, the ambulance will rush the organ to the recipient. Generally, the ambulances have their lights and sirens on when transporting organs due to their small window of opportunity for the recipient.
For the cost of the ambulance, that all depends on the state they are affiliated with, whom the insurance company carrier for the patient is, and how long of service they were used for. In some instances the patients insurance company does not cover the cost of the ambulance. And other insurance carriers will only cover the cost of the ambulance is the patient is admitted to the hospital for treatment.
Ambulances are a viable part of our medical health care system. Without the ambulances and their professional staff, many people would have died long before they reached a hospital.