I think that Steinbeck uses Lennie's contributions to represent how America and its social setting should operate, as opposed to how it actually does function. Lennie's primary contributions are that of companionship and loyalty. One of the most striking elements about Lennie and George is that they appear as a duo, as two people who stick together and remain loyal to one another. This is something that Slim and others point out as being rare. Lennie provides the glue that enables this to happen. Part of this might be due to the promise that George made to Lennie's Aunt Clara to ensure this, and another part of it might be how evident Lennie's dependence on George actually is. Yet, Steinbeck might be suggesting that the difficulty of "hard times" can only be successfully endured when people embrace a sense of collectivity and the idea that individuals have to remain loyal to one another. Steinbeck seems to be saying that while George does do most of the "heavy lifting" in terms of guiding their friendship, the only way they are both going to accomplish their goals is by working together and relying one another, something that Lennie brings to the table in plentiful proportions. It is here where the significance of Lennie's contributions is on full display in Steinbeck's mind and work, representing the idea that individuals must foster bonds with one another and advocate the idea of attachment to one another as opposed to severed isolation from one another.