Sorry if this question is strange, but why do I need to change my PVC valve if it's a closed system?

 

Not really a strange question...

The PCV valve sucks air out of the crankcase, and that air can be full of oily vapor or even oil mist. That stuff can make an oily sludge inside the valve and then the little plunger inside the valve doesn't move freely. Cleaning it might be an option, but it probably isn't a very expensive part.