Contrast a first-in unit that can do it all with many departments' specialty apparatus, which have their own problems. Many departments have specialized rigs to the point that they are all single-purpose apparatus. You'll see tankers that can't self-fill. Crews have to decide before every call which fire truck to take. First-in units might not carry enough hose to reach the closest hydrant.
Crews drive apparatus without floodlights and generators.
On the fireground, a crew might notice that the needed ground-ladder is still back at the station. If the foam rig doesn't arrive, the attack has to wait. Lots of engines have to rely on a tanker that might not respond to support them on the initial attack. On scene, if the closest pumper doesn't arrive with extrication or water-rescue gear, it will have to call for a specialty rig. I've received several letters reminding me just how understaffed we are in the fire service.