Whether or not Steinbeck intended it, the message a reader, and particularly a young reader, should get from Of Mice and Men is that uneducated and unskilled men will always have a struggle to survive. They have nothing to offer but their hands and muscles; and their marketability declines as they grow older and lose their vitality. The millions of men wandering the country looking for work during the Great Depression were mostly like the characters in Steinbeck's novel. When a person is in school, he or she has a golden opportunity to learn skills that will assure him or her of a job or even a profession. There are many opportunities for educated people after graduation. The most basic skills needed are reading and writing, which most of Steinbeck's characters lacked. Their reading consisted of cheap pulp magazines that passed from hand to hand. Poor Crooks was trying to educate himself with reading material salvaged from the junk heap.
The secret of success in any society is to know how to do something that other people want done. As an old Roman saying expresses it: "A useful trade is a mine of gold." Many of the occupations available to unskilled men in the 1930s no longer exist because it is easy for machines to do the same kinds of work. There used to be numerous jobs for elevator operators, newspaper vendors, pick-and-shovel workers, assembly-line workers, and pinsetters in bowling alleys. Even the backbreaking work the men do in Steinbeck's novel, lifting hundred-pound bags all day in the hot sun, has been taken over by machinery, just as all those teams of horses have been replaced by tractors with more power than a hundred horses.
Steinbeck himself was one of those workers when he was young, but he realized the importance of education and became a famous writer who was awarded the Nobel Prize in Literature.