By seeking out novelty, someone could be a more effective forager or better able to spot predators/prey hiding. ADHD often comes with a high attention to detail, so there's lots of advantages that could come with that (e.g. that twig wasn't broken before, something has passed through here).

Also, having different sleeping patterns can mean that the people with ADHD are likely better equipped to keep a fire burning overnight and possibly be alert for predators.