There is no real protection factor from the puck itself. Most racing suits typically have internal padding / armor to protect the hard points (knees, elbows, shoulders) I have lost pucks when crashing so don't count on them for protection. Really they are there to protect the leather for if/when you do get your knee down. You are correct in its a gauge to feel how far over you are. There are certain turns on certain tracks where the knee puck will be buried into the asphalt pretty good which can lead to accelerated wear but other wise you touch your knee down, gauge how far leaned over you are, then pick it up slightly.
For what its worth, I recommend Klucky Pucks for strength of the mounting Velcro and the longevity of the plastic they use.