The True Ancestors didn't have a specific reason in creating Arcueid. They normally only mold new True Ancestors to fit specific needs. There was no need for a being as strong as her, but in the end, she originated from nothingness, simply as a product to realize the pointless goal of "the strongest True Ancestor."  They used her great strength to make her into a weapon, the executioner of the fallen True Ancestors and of the most powerful Dead Apostles . According to Ciel , this happened in the 12th century AD, when the number of fallen True Ancestors was very high. She was able to perform this duty splendidly, but she was nothing more than a tool. She was taught nothing beyond what she needed to know to hunt her targets, to the extent that she even ignored the fact that she was the same kind of creature as those she was supposed to destroy, and went to sleep after every kill, with her accumulated memories erased.