At first, by being aware of the numerous sins that a careless person is likely to transgress, one will make the effort to draw away from these sins and his evil inclination will be weakened, eventually being cleared completely of this sin.
An additional benefit to learning these laws: should he stumble in this area, he won’t take it lightly, and he will not be considered a habitual speaker of lashon hara, whose sin our sages compare to the 3 cardinal sins (idolatry, immorality and murder) and who will not merit to greet the Divine presence.
At the very most, he will know that he has sinned and he will repent. He will ask his friends forgiveness and do his best not to transgress this law again.