Potentially when you connect to Public WiFi without going onto VPN anyone else on that network could see:
Every site you visit
All information you send across the web (i.e. email or facebook messages)
You never know who is on a network and if they are snooping other users data.
VPN is your best friend while travelling and using public WiFi connections, prior to doing anything on a new WiFi connection it should become second nature to first connect to VPN. This encrypts all your outgoing traffic preventing other users on the WiFi seeing what you are doing.