(Why) did the Buddha teach rebirth?

1. Was rebirth introduced in the First or Second Turnings, and why did the Buddha teach it? E.g. was it necessary to understand emptiness, causality, etc.? For example, I imagine that some "reasons why" might include: - Maybe it helps deemphasize a person's current lifetime, highlighting its impermanence, so that one does not cling to it or themselves. - On the other hand, for some, it seems to be a source of inspiration to persevere on the Path, for a "good rebirth". 2. Would any of the Buddhist doctrines, excluding karma and rebirth, fail to make sense without rebirth?