There are precisely three WiFi networks ...
... where VPN is not always necessary
1. Your own home network
2. Your employer's network
3. Your tethered mobile's network
However, even in these cases there are good reasons to VPN.
1. You don't want your ISP to track your internet usage
2. Both 1 and you may want to go round an e.g. Websense box ("No, boss, I'm not on Stack Overflow being 'social', I am taking self-directed action to learn the stuff I can't get from the company's $0/yr training budget"
3. Both 1 and you may want, $DEITY forbid, to look up the location of a pub on your PAYG mobile without having to take a passport, driving licence or credit card to the mobile store to prove that you are over 18.