The ancient Greeks and Romans built monumental structures like the Pantheon and the Parthenon primarily as temples and religious sites dedicated to their gods. These edifices were intended to honor deities, facilitate worship, and demonstrate the civilization's wealth, artistic skill, and cultural values.
While some monumental architecture served other purposes, such as housing government functions or offering defensive capabilities, the primary motivation behind the construction of the Parthenon in Athens and the Pantheon in Rome was their religious significance. The Parthenon was dedicated to the goddess Athena, the patron deity of Athens, while the Pantheon was built to honor all the gods of Ancient Rome.
In summary, the main reason for their construction was to provide a space for worship and to symbolize the civic pride and religious devotion of their respective societies.