VPNs
A VPN isn’t a bad thing but the way in which they are usually advertised to people are. A VPN isn’t for the purpose they advertise. What these companies are advertising is a glorified proxy.
Logging
They can and probably do log. Any that say they don’t could just lie. They can’t acquire everything but they can collect a reasonable amount of information.
Security and Privacy
A VPN by itself doesn’t bring you security. They may give some level of privacy but that is only in a few cases.
If you want it so you have an encrypted connection for something that expects plaintext, using a VPN just moves the problem rather than doing much about it. The VPN still handles plaintext. The VPN being able to see unencrypted traffic means they can also modify it before sending it to you.
Using a VPN doesn’t mean trackers can no longer match you to be the same person before you turned your VPN on. An IP address is on not the only metric. This can range greatly such as useragent and fingerprinting profile. Test yours.
Why Do They False Advertise?
They are specifically seeking out those who are looking for privacy and may have interesting traffic either because they are a honeypot or some other kind of malicious actor. It usually ends up getting people that are not very tech savvy.
When To Use a VPN
You are on a known hostile network (ISP MITM or public wifi) or you want to hide your IP from a non-government-sanctioned adversary like bypassing a IP ban for a service or avoiding anti-piracy letters.