What do physicians consider when referring patients to an imaging center?

Recently, I asked a nurse on staff at a GPs office what criteria the practice considered when referring a patient to an imaging center. This was her response (I replaced the name of the imaging center with XXX):

“We refer mostly to XXX because they have multiple locations, you don't have to call to get a stat Xray (usually), they are quick w/reports and they have premade scripts. We will write out other scripts to other places but only at the request of the patient –XXX  does a lot even chemical stress tests and when we have trouble getting precerted for them they will help. That's it in a nutshell.”