Faustus originally asked Mephastophilis to bring Helen of Troy because he wanted Helen (to own her). She is a symbol of beauty, and his hedonistic side wanted to satisfy his needs.
Faustus lusted after Helen and wanted to possess her.
Helen of Troy (Greek mythology) is the most beautiful woman in the entire world and the cause of the Trojan War. Before Faustus asked Mephastophilis to make Helen his lover, scholars asked Faustus to summon her.