It is especially important to bet for protection when there are multiple opponents. For example, if your hand is presently the best, but each of four opponents has a 1-in-6 chance of beating you, the four combined are actually a favorite to defeat you, even though each one is individually an underdog. If you bet, some or all of them will fold, leaving you with fewer opponents and a better chance of winning.
The term protection is also often heard in the context of an all-in player (see Poker/Table stakes rules), because a bet by any player serves to protect the hand of an all-in player just as it protects the bettor (and possibly moreso). To deliberately make such a bet solely to protect a hand other than your own is a form of Collusion.