Preseason polls should exist because they're fun. If we want to get all haughty about polls that "matter," the only poll that technically "matters" (outside of soft issues like brand, prestige, etc.), is the final CFP poll before the playoff. The rest of the polls are just fun for fans to goof off with and argue about.
FWIW, I think the polls (ap/coaching/playoff) all matter bigtime because it's more fun if you treat them that way. And I love polls -- especially preseason polls because they're fun!