The answer is, because we need it for all other properties. Long range order is measured by diffraction experiments. The number of scatterers in a small volume element is . The density is real if no absorption takes place. Each scatterer at position is the source of an outgoing spherical wave.
Inelastic processes are also important, but have to be discussed much later.
We ignore multiple scattering. Next we assume that the detector is far away compared to the size of the sample. In that case we can define a vector with magnitude k and in the direction of the detector. We take our origin at the center of the sample. We then have for all vectors in the sample.