DO's "private" networking was not even truly "private" previously as it was shared among its customers. Only recently did it get to the point that the "private" network is separated from the rest. Anyway, even the new "private" network does not allow for something like installing a custom DHCP server and configuring custom subnet for the nodes inside. One of the most common use cases is to route outbound traffic from all the nodes inside a private network through a public gateway and DO's current configuration does not allow that.