IPSec is one. You would burn two IPv4 to enable IPSec between two machines you control. That's a no brainer with IPv6. It is cumbersome and expensive with IPv4.
And given that most datacentres (outside of cloud services) are IPv6 enabled, this makes sense to secure server to server communications.
And given that most datacentres (outside of cloud services) are IPv6 enabled, this makes sense to secure server to server communications.