Does VPN-NAT improve security for iOS devices using VPN?

That is, would any unrequested inbound traffic entering via the VPN tunnel be able to do mischief given the iOS sw and hw design and the security measures Apple uses in its App store re iOS app? (I understand the jailbroken iOS devices are at risk and should use NAT or an app like Firewall IP.)

I see VPN providers offering that as an add-on service, but don't yet see the value in it for iOS devices despite general recommendations for it.

Thoughts?