I'm a new nurse (clearly) and I'm still figuring out why normal saline is hung alongside a primary line? Is it just to do the initial priming of the tube so you don't waste what's in the primary bag?
The biggest advantage I can think of is pre-flushing and post-flushing after medication administration. Say you're running a primary fluid that's incompatible with some of your medications, but you know the meds are compatible with NS. If you have NS hanging already, you can stop the primary fluid, flush the line with the NS, give the medication, and then flush again with NS before resuming the primary fluid. All without switching tubing or fluid bags.
The other benefit could be that you have NS readily available in the event that the patient starts going bad and needs to be bolused quickly. While we use D5 0.45% NS for maintenance fluids as standard in our PICU, I keep NS close by for critical patients, shocky patients, sickle cell crisis, or others who might need quick fluid resuscitation. If you've got a patient whose BP suddenly starts trending down, you can respond much more quickly if you already have the NS hung and just switch infusions vs. going to the med room, grabbing the NS, spiking the bag, priming, etc.
I have also had patients that were getting frequent IV meds and we would just leave the NS running at 30 TKVO ( to keep the vein open)so we didn't have to keep flushing and locking it in between.
I would say the main reason is just as Ashley stated, to provide a flush in betweem meds and to have a line in if you needed one in a hurry.If a patient's BP all of a sudden bottomed out you would be hard pressed to get an IV in.Already having one could make a huge difference.