Having a VPN is more of an 'identity and data protection' rather than unblocking websites. It's a perk though, but what you should understand that VPN encrypts your Internet data from the get-go. On the other hand, HTTPs only protects when a trusted handshake is established, even then you are not safe. Before that, your data is open for interception by an attacker, government, or any other stakeholder. Count your ISP in for accessing your data and using your information against you.
Since, your mobile device is more like your identity, having personal information and other private data. Protecting such data is essential. You wouldn't want any third party marketer to be following your social profiles? Same is the case with public shared Wi-Fi that are accessed by hundreds of people. You never know who has technical skills to access your data on public network and pretend to be you on social profiles, using small files such as cookies. So, it is best to have a VPN on iPhone rather than relying on HTTPs alone. I would say security is process and you need a combination of tools to protect every loophole possible.