What are the benefits of requiring HTTPS between AWS CloudFront and the origin server?

If an attacker is able to put themselves between your origin server and CloudFront edge location - it won't be so difficult for the attacker to capture your HTTP traffic and extract your information. Theoretically, it can be done on the side of your ISP. It is called as a "Man-in-the-middle" attack: https://en.wikipedia.org/wiki/Man-in-the-middle_attack

If you trust your ISP or hosting provider for 100%, you don't need HTTPS. Also, there is no need to encrypt public content. Anyway, the ultimate best practice is to use HTTPS always.