Medea helps the cause of women by showing that, even in a male-dominated society, where women have no rights whatsoever, a woman can still show the strength of character to take control of her destiny. Abandoned by Jason, and having left behind her own family, Medea is effectively a non-person, a stateless refugee. As a woman without a family support network to fall back on, she's quite literally on her own.
This would be a huge challenge for anyone, let alone a woman living at a time when women were totally dependent on their male relatives. Yet Medea rises to the occasion, channeling her rage at Jason's betrayal into taking control of her life in a way that would be unthinkable for most of her female contemporaries.
However, the way that Medea chooses to do this is problematic, to say the least. This is because exercising control over her life revolves around causing death and suffering to others, most notably her own children. It says a lot about ancient Greek society that a woman can only act independently by engaging in such murderous acts. Medea's transgressions against society's norms may not exactly redound to her credit, but they don't say a lot for society either. In that sense, Medea's actions may undermine the cause of women, but they also lay the foundations for successive generations of women to take control of their lives, albeit without necessarily resorting to murderous violence and infanticide.