First and foremost, using a VPN prevents anyone on the same network access point (or anywhere else) from intercepting your web traffic in a man-in-the-middle attack. This is especially handy for travelers and for those using public Wi-Fi networks, such as web surfers at hotels, airports, and coffee shops. Someone on the same network, or the person in control of the network you're using, could conceivably intercept your information while you're connected.
All VPNs have to do some kind of logging, but there are VPNs that collect as little data as possible and others that aren’t so minimalist. On top of that, some services discard their logs in a matter of hours or days while other companies hold onto them for months at a time. How much privacy you expect from your VPN-based browsing will greatly influence how long you can stand having your provider maintain your activity logs—and what those logs contain.
Even if none of the above really sound right to you, you can still benefit from using a VPN. You should definitely use one when you travel or work on an untrusted network (read: a network you don’t own, manage, or trust who manages.) That means opening your laptop at the coffee shop and logging in to Facebook or using your phone’s Wi-Fi to check your email at the airport can all potentially put you at risk.
Typically, when you try to access a website on the Internet, your ISP (Internet Service Provider) receives the request and redirects you to your destination. As your Internet traffic passes through your ISP, they can see everything you do online. What’s more, they can track your behavior and sometimes even hand your browsing history over to advertisers, government agencies and other third parties.
If you are depending on your VPN to keep your activities even mildly anonymous, you need some sense of security that the VPN isn’t just going to go down and dump all your traffic out into the regular internet. What you want is tool known as a “kill switch system”. Good VPN providers have a kill switch system in place such that if the VPN connection fails for any reason it automatically locks down the connection so that the computer doesn’t default to using the open and unsecured internet connection.
That attitude to the safety and privacy of personal data creates an enormous online security risk. Public Wi-Fi networks, which are ubiquitous and convenient, are unfortunately also highly convenient for attackers looking to compromise your personal information. How do you know, for example, that "starbucks_wifi_real" is actually the Wi-Fi network for the coffee shop? Anyone could have created that network, and they may have done so in order to lure victims into disclosing personal information over it. In fact, a popular security researcher prank is to create a network with the same name as a free, popular service and see how many devices will automatically connect because it appears safe.