When you blow out the lines using a compressor and blow out plug installed on the city water hookup inlet port. it clears out the lines so your rv antifreeze can do its job without being diluted, full strength, max protection.
Some people just use the air to blow out the system but some take the extra antifreeze step.
Some people say that they do NOT like to use the antifreeze because it sometimes leaves an oily film in the lines that has to be flushed out when you go to use the rv again.
If that becomes an issue for you, you could always blow out the antifreeze right after you pump it thru your lines. At that point, the last fluid thru the lines would be antifreeze at full strength, there is nothing to freeze in the lines anymore, your lines will remain protected.
Just be sure to turn off the water pump switch right after you pump the antifreeze thru, when you're all finished.
IF you accidently leave the water pump on and flip the petcock on the antifreeze pickup tube, the pump will suck the little bit of remaining water that might be in the bottom of the freshwater tank or line. That water will sit in the pickup tube petcock and crack it if it freezes.
Your pex water lines are very freeze resistant but we all must take extra care to protect the plastic petcocks and elbows. Plastic toilet parts as well.