A VPN (Virtual Private Network) creates a secure pipe/connection between you and the internet. It’s most useful when you are using a public wi-fi network, which is more velnerable to man-in-the-middle attacks and other kinds of malicious activity. Some people also use it when on a cellular network. Hacking on cellular is possible, but requires more expertise and specialized equipment.
There is no special reason why an iPhone X needs a VPN. It is no more, nor less, secure to this kind of attack than other phones, tablets, or computers. If you do sensitive work on your iPhone (banking, sensitive communications, or confidential work) while on public networks, a VPN is highly recommended while on those networks. If you or your job requires a very secure phone, then you may want to use a VPN at all times.
In the case of work security, many companies have their own VPN servers, and will mandate their use for work related devices.