Part #2
There are many valid reasons to use a VPN. Contrary to frequent assertions that you’ll encounter, using VPN over public networks does provide useful and significant protection against local attacks and traffic monitoring which are endemic on public networks. For this reason alone, it may argued that using a VPN reduces (but does not fully eliminate) avoidable risk.
Some contributors are correct in there assertion that, where used, a commercial VPN operator has visibility of your network traffic - as your network traffic is obviously being routed via their VPN gateway/endpoint. Whilst your VPN-tunnelled traffic is protected from locally prying eyes of the open WiFi network to which you connect, your traffic is ultimately delivered to the internet from the VPN gateway in its original (partially encrypted) form. As such, unencrypted protocol traffic is protected from interception on the high risk “open” WiFi network, but can [technically] be seen by the VPN gateway.
To reiterate, traffic visible at the VPN Gateway/endpoint is still partially encrypted at protocol level. As such, for practical purposes, the traffic exposed to the VPN Operator is no more at risk than would otherwise be exposed to your Internet Service Provider - but is fully encrypted by the VPN tunnel over the “least trust” open/insecure WiFi network. If the VPN Provider is chosen with care, risk of traffic interception over high-risk networks can be significantly mitigated.
For this purpose, use of a VPN is a “trust” exercise. In whom do you place greater (dis)trust? The open/insecure WiFi network to which you make your network connection (with all of its consequential risk, potential traffic monitoring/interception and alteration), or the VPN Operator? Which carries greatest risk to you, the security of your network traffic, or your privacy?
A reputable VPN Operator (noting that “free” services are generally outside of this category) has no commercial interest in your network traffic - but may be bound by legislation of the country in which it is based to collect metadata concerning your connection. The latter you can nothing about - and unless you yourself engage in nefarious activity, should offer no concern. The former simply requires wise selection of your network operator - often requiring parting with money on subscription terms.
If the user has the technical capability (and competence) to correctly configure a VPN endpoint/gateway, trust in the VPN moves from that of a commercial VPN Operator to the end-user entirely - removing any perceived issues with the VPN Operators interest.
To conclude, whilst the explanation is necessarily technical, I hope to have provided additional qualified information as to some of the benefit (and limitations) of using a VPN.
In summary, when using an open (public) WiFi network, a well chosen commercial VPN offer significant benefit to both security and privacy.