This is SOP (Standard Operating Procedure).
1. Imagine a scenario where an ambulance arrives as a first responder, and the EMTs [1] find that they cannot gain access to the emergency scene, because the home or other structure is locked or otherwise blocked. In this case, a firetruck would need to be dispatched, because it deploys with the correct equipment (e.g., Halligan bar [2], Denver tool [3], etc.) and personnel trained to handle such a situation. Also, ambulances are typically deployed with only two personnel, and that may not be enough in situations where items need to be moved or cleared to gain clear access to the emergency scene.
2. Most firefighters are cross-trained as EMTs. If they are local to the emergency scene, then they may be the first responders, while an ambulance is en-route.
3. Incoming 911 calls can often be confusing, with callers under extreme stress, experiencing situations that they have never faced in their lives. Imagine a situation where a caller is so distraught, overwhelmed, or focused on a life-saving effort, that they cannot articulate that they need firefighting support, in addition to an ambulance.
The bottom line: in an emergency situation, it is better to have more help on-hand than too little.
[1] https://en.wikipedia.org/wiki/Emergency_medical_technician