What was steinbeck's purpose for including the license plates and car types of the vehicles driving west on highway 66?

This is to separate the migrant workers from the steady customers of diners who can pay for meals and products. For the people who own the diners and other small businesses along Route 66, the migrant workers are little more than a burden on them, asking these people, who are simply attempting to make a living, for handouts and charity. The men and women who work at the diners on Route 66 view the migrant families with a conflicting sense of loathing and compassion. Hope this answers your question!