[nhaehnle]

In the end it's a matter of convention. If you think about Newton's "model" of gravity, for example, you'll notice that the formula that describes the gravitational force can be plausibly explained based on intuition. However, the gravitational constant (i.e. the parameter) has no explanation. It just is.

Of course, a great deal of physics is ultimately about trying to make the parameters go away by explaining them using more fundamental models. But at any given level of abstraction, you'll have parts of the model that are reasoned intuitively, and parts of the model that just are the way they are, for no good particular reason other than that's what you happen to get by measuring.