Use of Criminal Rating in San Andreas

The Criminal Rating is a rating level in all games in the GTA III Era based on the criminal activities of the player. In Grand Theft Auto: San Andreas, the criminal rating can be viewed in the Stats section in the pause menu. A new game starts the criminal rating at 0. GTA San Andreas did not feature the Media Attention statistic like Vice City did.

--Wiki located here

It basically is just a "How evil are you" and provides no positive or negative effect. Just the satisfaction that running over multiple people at a time gets you that much closer to being the Godfather. Source