Mary, along with Abigail Williams, initially accused Tituba of witchcraft, and thus set the stage for the entire witch hunt.
How her actions affected the plot
The act of accusation triggered the hysteria that engulfed Salem and threw the community into chaos. Also, Mary's initial support of Abigail and her willingness to participate in the "witchcraft" accusations lends credibility to Abigail's accusations.
Mary's eventual breakdown and confession expose Abigail's manipulative tactics and the fear that fuels the accusations. This revelation casts doubt on the validity of the trials and adds a layer of moral complexity to the situation.