Is a VPN necessary for device security in public spaces?

Hardly.

Unless the VPN is provided by your employer or school, it's no more secure than being without it as you cannot prove that whoever handles the VPN will not misuse your traffic in any way.

You're just changing who can see your traffic and assuming the provider's of the VPN can be trusted.

No. All data to and from the device is already encrypted; a VPN is only needed if you're part of a business or school which says so.

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In addition to the excellent advice already given, you might want to read this:

VPN: What you need to know - Apple Community

And this:

Don't Use VPN Services