In many hospitals, the part time staff have lower pay per hour than the PRN staff, making it less expensive for the hospital to give those hours to the part time people who want them.
Also, part time employees usually make a greater committment to the hospital -- committing to work a certain number of hours per pay period, a certain number of holidays, etc. ... a committment that is greater than the committment made by PRN staff. Some hospitals like to reward that greater committment made by the part timers by giving them the first chance to work any extra shifts available. (especially if they are cheaper than the PRN staff)
So ... from the hospital's point of view ... it may be preferable to give those hours to the part time staff first.
Similarly, when cancelling people due to low census ... The more expensive staff members may be cancelled first. (Anyone working scheduled overtime... then the PRN staff ... then the part time staff scheduled above their budgeted hours ... then finally, people working within their committed, budgeted hours.)
When you are thinking about administrative practices, you can't forget to think about the financial aspect of it. Anyone considering a job should be sure to find out the facility's policy on such things before they decide whether they want to be full time, part time, or PRN. Each type of position has its advantages and disadvantages -- and they can vary from place to place.