Open borders provide a way for migrant workers to move from one country to the next in search of seasonal jobs. Since the jobs they fill are often low paying farm work that most people do not want, they are not taking jobs from locals but rather filling a need for short term labor. This is not only the case in the United States but in many countries throughout Central and South America.
To put it in the most simple terms, migrants who are crossing the border would not have to worry about breaking the law if the they had open borders to cross.. They could travel with their entire families and would not have to risk their lives in order to enter this country.