That's because your lowest floor is below the level of the nearest manhole. If the city were to back up, it would end up in your house instead of the street without that valve.
If the check moves freely and the mating surface of the check and the valve body look smooth, I wouldn't worry about changing it. The ones we put in are plastic, so i'm not sure if you'd have access to the exact same model that you have there. If you were to get a different model, it may change the dimension, and cause you to have to get the jackhammer out.
Which direction is the slope of the pipe going?