Why when I call 911 for a medical problem an entire engine or truck company comes along with the ambulance?

In many situations, depending on the severity of the call, a two person paramedic ambulance will need extra assistance for manpower. If for example the emergency is a cardiac arrest, we need a member for chest compressions, one member to ventilate the patient, one member starting IV’s, another intubating the patient, and yet one more talking to the hospital advising them of our patient’s status. Therefore we always dispatch another unit along with the ambulance and if the situation can be handled by the ambulance without any assistance, then the engine or truck will immediately go back in service for other calls.