In a sense, Steinbeck does seem to emphasize that dreams are not meant to be achieved. George and Lennie have a dream of owning a home. That dream is never achieved. Candy joins in on George's and Lennie's dream, but he realizes by the end of the novel that his dream will not come true. Also, Curley's wife dreams of becoming an actress. Obviously, that dream never comes to past.
Steinbeck could be pointing out that some dreams are not realistic. There is a dream that is too big to achieve. Perhaps, Steinbeck is pointing out that dreams are just that--dreams and nothing more. Nonetheless, George and Lennie had a dream that helped them by having something with which to look forward.
I believe Steinbeck included the dream to give hope in a hopeless society.