VPN service - is it recommended if I only travel overseas twice a year?

Unless you need to connect to a company or school that requires use of a VPN, you probably do not need such a thing unless you're in a profession or situation where there's a high likelihood that someone would be tracking you and trying to intercept your communications. Very few people, relatively speaking, would fall into that category. I would, however, suggest that you avoid accessing bank accounts or other financial services unless absolutely necessary, particularly if you plan to be using Wi-Fi for data rather than your iPhone's cell connection. While the likelihood is small, there is always the chance that some bad actor could be scanning such Wi-Fi networks trying to capture data. Any responsible financial institution will be using encryption on their connections, but you never know. And of course you never want to put such information into an email or standard text message; those are never secure.

Regards.

Essentially it all depends on the reason you believe it is needed. Simply traveling does not necessitate the use of a VPN.

Could you go a bit more in depth?

In addition to what varjak paw explained, you may find some useful information here:

Don't use VPN services.