Universities rely on poorly paid graduate student and adjunct labor for work like grading and the teaching of introductory level undergraduate courses. This is the grunt work of academia, the work that scholars eagerly throw off if they are lucky enough to get a tenure-track position. The system has always been unjust and precarious and has only become more so in recent decades, when three-quarters of all instructional jobs filled in American universities have become nonpermanent positions.
Graduate student unions seek to restore power and control to this class of workers. But having a union to back you up is the exception rather than the rule for graduate students in the United States.