The only real reason to do this is in busy ERs, where the patient may not have close nursing care. If you get the air out and you come back 4 hours later and the liter finished 45 minutes ago, there is no air in the line. You could do something similar by spiking the bag and holding it upside down, squeezing the air out and then turning it rightside up and flushing the line. No air in the bag, no problem with contamination.