This article understands why you should use VPN on public Wifi.When you are connected to public Wifi, you would connect to the nearest wireless transmitter / access point using the credentials provided by the wifi service provider. You would connect, with or without a common password. On a wireless channel , there is an option for access with or without encryption. Encryption ensures that all data transmitted through the wireless channel is encrypted.
If the service provider is using an open system where the data is not encrypted, an attacker who is also connected to the network can snoop all data which is sent from your system, simply because both you and the attacker is connected on the same network. If the wifi service provider is using encryption , this can also be cracked by an attacker on the network making you vulnerable.
How does a VPN help on Public Wifi
When you install a VPN , all traffic sent from the device is encrypted using a session key assigned by the VPN service provider and not the wifi service provider. So even if the attacker has gained access to the wifi key, he would not be able to access the data sent from the device , since the device would be using a different encryption key assigned by the VPN software.
So by using a VPN on a public Wifi , you would have an additional layer of security.