While he is a formidable opponent, he's not a gym leader or a member of Team Galactic. Defeating him is mostly about advancing the story and uncovering his mysterious motives.
Here's what you gain from defeating him:
* Story Progression: Beating the Ruin Maniac is necessary to continue the main story and move on to the next challenge.
* Items: He drops the Iron Ball which is a held item that reduces the user's speed by half.
* Understanding: The fight helps you understand the Ruin Maniac's obsession with ancient artifacts and his connection to the legendary Pokémon Uxie.
So, while you don't get any specific rewards or badges, defeating the Ruin Maniac is crucial to the story and unlocks new areas and challenges in Pokémon Diamond.