I think zeroTier is meant to be easy whereas openVPN is versatile and can be configured in many ways. One use case of a VPN is to connect two local networks to

I think zeroTier is meant to be easy whereas openVPN is versatile and can be configured in many ways. One use case of a VPN is to connect two local networks together. Another is to have your traffic appear to be coming from a different geographical area. In some cases you wouldn't want the local networks to be able to speak, like commercial VPN services, for security reasons.