You leave the faucets closed in the event that some connection suffers a leak below the faucet, which would slow the migration of the pink stuff to that point. Here is the physics; "if" you do incur a leak, and your faucets are closed, each amount of fluid starts to create a small vacuum until air seeps into equalize the enviroment. Look at it this way, gas cans have a vent to allow the fuel to move freely without chugging. Same principle that you're trying to prevent, in case of unexpected leak in plumbing. Just sayin' Rocket science, no, precautionary, yes.
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