Q gives Riker godlike powers as a test of character.
Q returns and challenges Picard to a game, offering Commander Riker the powers of the Q Continuum. Riker is granted godlike abilities and, tempted, uses them to give gifts to his crewmates. Q wants to learn whether humans can be trusted with such power.
The episode presents Q as a more playful and malicious trickster than in the pilot, and it sets the stage for his many later visits to the Enterprise. It is also an early test of Riker's character, since his final choice about the powers is a quiet statement of who he is.