What can the old Star Wars Expanded Universe give to the new canon?

Star Wars’ big blue bad is back. Disney announced this summer that Grand Admiral Thrawn, one of the best-known villains from Star Wars’ past, would be reentering the official canon. This past weekend, he made his triumphant return with the premiere of the third season of Rebels.
Thrawn is notable because he's one of the only major characters from the Expanded Universe (EU) to make the leap between the previous Star Wars canon and Disney's new one. But why does that matter? Star Wars