Dust Caps: Why are they so important?
When a regulator is rinsed off after a dive, it is very important to ensure the dust cap is in place. The dust cap ensures water cannot enter the first stage. Water entering the first stage may carry salt down the high pressure hose and into the submersible pressure gauge. The salt then crystalizes and prevents the pressure gauge from showing accurate pressures.
Remember the "Happy New Year" whistle that uncoils when you blow it? That is very similar to the internal workings of a submersible pressure gauge. Imagine spitting sunflower seeds into the whistle when you blow it...when you stop blowing, the whistle will not recoil as it did before...likewise, the pressure gauge may show that you have pressure in your scuba cylinder when you really don't, which is a very dangerous situation.