Well, an automotive engine creates a vacuum in it's intake. We can use this vacuum to operate accessories like cruise control, door locks, or headlight shutters. We use rubber lines to pipe this vacuum around to the various accessories and sensors. With time these rubber lines start to degrade, crack, and leak, causing said accessories not to work and poor engine performance. An easy test is to rub your fingers along them, if it comes off on your skin, the lines need to be replaced. Reason the ladies get moist over fresh vacuum lines is because no one is getting laid if your hooptie won't idle when you get to the make out spot.