Final answer:
In the story of Perseus, Mercury (Hermes) is essential for providing Perseus with winged sandals that allowed him to fly, aiding in the beheading of Medusa and enabling further heroic adventures.
Step-by-step explanation:
The importance of Mercury (Roman counterpart of Hermes) in the story of Perseus primarily revolves around his role in aiding Perseus to complete his quest to behead Medusa.
Mercury provided Perseus with winged sandals which enabled him to fly, an essential attribute that allowed Perseus to avoid Medusa's petrifying gaze by looking only at her reflection in Athena's shield. With these sandals, Perseus was not only able to approach Medusa safely but was also capable of performing subsequent heroic deeds, such as rescuing Andromeda.