What does Kevan give to Tywin that he doesn’t already have?

Well, Kevan’s a rare commodity for Tywin – a truly trustworthy subordinate. He’s completely loyal to House Lannister, both by birth and by inclination, and Tywin can trust him to act for the family in ways that he can’t your Paynes, Cleganes, or Lorches. 

Moreover, Kevan is a careful, cautious, and steady subordinate. He won’t exceed orders and kill everyone like your Gregor Cleganes or your Amory Lorches. He won’t get ideas into his head like Tyrion or go charging off like Jaime. He’ll do exactly what’s asked of him.