Overall, Curley's wife is one of the most unfairly treated characters in 20th Century American literature. Certainly she is the loneliest character on the ranch and, despite her abhorrent threat to have Crooks strung up, her actions throughout the novel depict a lonely woman desperate for attention. Instead, because the limited third-person point-of-view only looks at Lennie's and George's perspectives, Curley's wife comes off as a "tart" who leads Lennie to break her neck. Overall, Curley's wife is symbolic of the way women might have been treated on a ranch in the 1930s.
Starting with her name, or lack of one, it's clear that Curley's wife is just that: a wife. She maintains no purpose on the ranch other than being the boss's son's spouse. In fact, Curley's wife doesn't even receive much respect from her husband, since he goes off with the ranch hands to a whorehouse. At the end of the novel, Curley doesn't seem heartbroken about his wife's death, but instead seems hellbent on lynching Lennie.
This lack of attention from her husband, or lack of any duty for that matter, leads Curley's wife to flirt with the other men on the ranch. Before marrying Curley, she clearly received attention. She explains that a man asked her to make films and she "Coulda been in the movies, an' had nice clothes..." However, she's left wandering around the ranch looking for attention, which none of the other men give her.
This role as wife leaves this woman without opportunities. In a novel about loneliness, she is the loneliest character, which is due directly to her role as a woman on a ranch.