They have different purposes. Think of Stories like refined Snaps: they should be humanizing peeks into the world of the account while retaining the almost magazine-esque aesthetic of Instagram.
Posts, on the other hand, can be more directed. These should have the highest quality and actively define the voice of the account. More brand heavy than stories.
Having too casual of posts will make your account seem tacky, while it may ingratiate your stories. Having too formal of stories will make the account seem like an ad machine that ends up annoying followers. Gotta strike a balance.