When people think of calling 9-1-1 for a medical emergency, they may focus mostly on getting to the hospital. Most research has demonstrated that rapid, on-scene medical intervention produces the best patient outcomes. The fire service is best positioned to deliver this critical care, as they can regularly arrive on-scene faster than the ambulance.
Other primary duties of the fire department are to respond to fires, hazardous materials incidents, auto accidents, etc. Quite often, these calls come in while returning from other calls. In order to maintain the fast response times to provide the highest level of care, the entire crew and equipment need to be together ready to respond. If a company needs to drive past an incident to switch into a different apparatus, the results could be devastating.
The bottom line – a better chance for patient survival, property conservation and/or environmental protection is why the fire department responds in their primary apparatus to medical calls.