Xplorer explained it very well, but just to summarize, a lot of major cities house only one engine and one ladder ( quint, tower, whatever you want to call it) and only 1-5 rescues per city. It may take longer and would pull a rescue out of service to handle a little accident with no injuries and just having to pop the door. With ladders carrying the extrication tool they an arrive on scene quicker and most often the engine will take care of the ems part of it, and like xplorer said, if needed a rescue can be called for more equpiment.