This is the way that MARCS is designed. It allows for communication from any part of the state to any part of the state. In a basic sense it is like the state's own private cell network.
As an example, If I were a Fire Chief in Northern Ohio and had traveled,say, to Southern Ohio and a major incident occurred, I could still monitor and communicate back from where I was at in Southern Ohio with out changing talk-groups.
That's the simple explanation.