This is all about being able to invoke AWS Lambda from an IPv6-only network. Their copy states as much: "(...) removes the need for expensive networking equipment to handle address translation between IPv4 and IPv6."

Use cases will be IPv6-only networks with no NAT64 capabilities, either due to compliance reasons or because you're running, eg, IoT devices that need to call home over a provided 3G/4G connectivity and you were only able to get competitive prices if you stayed away from the cellular provider's IPv4 stack.