Here's the reason why Dinesh Karthik wears a unique Helmet?..

His helmet has a different shape, similar to a baseball helmet.

The helmet has unique strap system & extra padding helps absorb the impact of ball.

The unique design provides more protection to the face and eyes.

Unique strap system allow him without worrying about  helmet slipping during play.

The rounded shape of helmet provides better protection to the back of head and neck.