George and Lennie may dream a little dream of owning a farm, but they don't get very far with their to-do list before it all crumbles in heartbreaking failure. As Crooks points out, all ranchhands dream of owning their own farm; it's their version of the 2.5 kids and white picket fence. Unfortunately, white picket fences are in short supply during the Great Depression, and Of Mice and Men ends in the only way it can: with the utter collapse of everyone's dream—even Curley's.
In Of Mice and Men, dreams are necessary, even if the characters know that they'll never achieve them.
Steinbeck seems dreams and foolish and unnecessary, just leading to more sorrow.