A par time answers a different question than a shot timer does. A shot timer listens, catches the moment of each shot and hands you a number afterwards. A par time hands you nothing at the end. It draws a border around the work, and the only verdict is whether you finished inside it.
That makes it the natural tool for dry practice. At home you are rarely chasing a number: you are trying to run a sequence cleanly, inside a fixed window, enough times that the movement stops asking for your attention. A border does that job better than a measurement, and it does not depend on sound at all. The click of a striker is quiet, it differs from pistol to pistol, and a room swallows it in its own way, while a par time simply runs on the clock - the same with a real firearm, an air pistol or an airsoft replica.
The number itself is personal. You find it by running the drill honestly at a generous setting, then cutting a tenth or two and holding the new pace for a series of repetitions. When the movement starts to fall apart, you have found today's edge, and that is where the work lives.