Not all counties are run like Wayne county. That was an agreement between the city and the county to have the Sheriffs primarily control the jail system.
In other counties the sheriff has a bigger scope because you have townships that don't even have a police force. In that case the sheriff steps in. Reminder, the Sheriff is the head law enforcement officer in his county. He supersedes all other police organizations, even state police because he is an elected official. They have jurisdiction anywhere in the county.
Oxford township has 5 police officers fulltime total. That area relies on additional support from local sheriffs to handle things outside the scope of the township and probably in the township as well.
Oxford itself is not going to have a SWAT team, they rely on Oakland County Sheriff just like almost all of Oakland county. Guess what.....they have a Bear cat.
So, for a situation like the school shooting. Lets say he had a rifle and didn't give up and held a few kids hostage. You are not going to roll up to the front door in a crown vic. You might need to use the bear cat for cover as you move to the building. It would also be a place safe from gunfire to transfer students to a safe place if they had to leave in a direction the shooter could see. It also can be used to push in doors or walls to gain access. It can act as a command center where communications are set up for a long hostage situation.
The point is that its not at your local PD doing doughnut runs at 8am. It's used for special situations where people have guns other than the police.