This point is a means of foreshadowing events to come in the story- namely Lenny's death. It is important that we know about the Luger and that it is easily accessible to anyone on the ranch because it comes into play later on. The men believe that Lenny is armed after he kills Curley's wife, which is helping to fuel their angry revenge and subsequent man hunt for Lenny who has run away. George is actually the one who steals the Luger in the final chapter of the book and he uses it to shoot Lenny so that the other men cannot torture him for killing Curley's wife. George does not do this maliciously, he does it out of the deepest sense of friendship a man can have.