I've been reading that a crawlspace needs to breathe ? Why ?

I understand that if moisture gets in it would need a way to get out - hence the vents. I guess that is what is called breathing for a material.

But if I prevent moisture from even getting in or have a different way of getting moisture out, like a dehumidifier, do I need a vent at all ?

I have the same question about our cement floor in the garage... What is the harm in applying some kind of garage floor coating... does it make the cement crumble if there is too much moisture ?