"She's doing that anyway. It's why she came to Westeros." To make her own claim on the throne, a better claim to it than Cersei has. The only reason Cersei still has the throne is because she refuses to give it up. She has no legal right to it otherwise. "It's a threat from the north. The reason the Wall was built in the first place, and the Night's Watch founded. If this threat takes over the seven kingdoms, it won't matter who wins the war."