Every language has idioms that appear repeatedly in speech and writing. From the strictly grammatical viewpoint, however, they don’t always make sense. Such is the American English idiom “You know what?,” which is defined in the Merriam-Webster Online Dictionary as:

1 —used to emphasize the statement that comes after it You know what? I never trusted her. She lied to me again, but you know what? I really just don’t care anymore.
2 —used to get someone’s attention Hey, you know what? I’m hungry.

Senseless idioms are a good example of the arbitrariness of language.