The death of Curley's wife sets off a chain reaction for the events that close out "Of Mice and Men". After Curley finds out that his wife has died, he becomes determined to kill Lennie gruesomely for revenge. This adds extreme tension to the story. While the initial reaction to her death is a manhunt led by Curley, her death eventually leads to Lennie's death at the hands of George. Her death also is a reminder of the incredible strength Lennie held and how incapable he was at managing it. The death of Curley's wife is also symbolically important and represents the futility of dreams. Curley's wife has big dreams of becoming a famous actress in the movies, but her death signifies the end of her dream. Her death also signifies the destruction of the American dream in general, since the consequences of her death mean that George and Lennie will never achieve their dream of owning a farm together and living off the land. Ultimately, her death is the catalyst for all of the unpleasantness that occurs in the latter part of the novel and is of huge importance to the plot and the novel's themes. However, it should be noted that neither the novel nor its characters seem to care too much about the loss of life itself; Curley's wife was merely the property of Curley, and trouble to everyone else. Her death is a tool to create the novel's climax and to trigger events for the male characters.
The death of Curley's wife is ultimately unimportant to the novel's outcome since it is clear that George and Lennie were never going to achieve the American dream and live on a farm of their own. Their lack of wealth, Lennie's lack of intelligence, and their negative past experiences in Weed all create a barrier that is impossible to breakthrough. If Lennie hadn't killed Curley's wife, he would have done something else to upset Curley, leading to his eventual death or the permanent loss of his and George's dream. Therefore, Curley's wife's death is not crucial to the plot.
[P1] The death of Curley's wife leads to a manhunt that places in danger. [P2] The death serves as a reminder of the incredible strength Lennie had and shows how he (tragically) misused it. [P3] The manhunt ultimately results in the death of Lennie.