An instance might want to reach anything on the internet that's using an IPv6 address is a use case. Also, having instances connect to IPv6 parts of the intern

An instance might want to reach anything on the internet that's using an IPv6 address is a use case. Also, having instances connect to IPv6 parts of the internet while you can see which subnet of your vpc the traffic originates from, and maybe filter on that fact. Having routable addresses is a different thing than them being public or private, which is determined by the fact if you accept traffic to them or not when it arrives at the last hop before entering your vpc.