What a VPN is and what it actually solves
The idea, without jargon
When you connect to the internet without a VPN, your traffic passes through whatever network is in front of you — the hotel wifi, your carrier's cables — and from there heads out to each site you visit. Whoever controls that network in between can see which servers you connect to.
A VPN changes the route. Your device opens an encrypted tunnel to a specific server, and all your traffic travels inside it. The network in between no longer sees the sites you open: it only sees that you are talking to a server and that the contents are encrypted.
It is a little like posting a letter in an opaque envelope instead of a postcard. The courier still knows you sent something and which office it goes to, but cannot read what it says.
The three cases where it matters most
Networks that are not yours. Free wifi at an airport, a café or a hotel is run by someone you do not know. You cannot tell what it logs or how it is configured. Encrypting your traffic there removes a whole category of problems.
Working away from the office. If you review company documents on a shared network, that connection crosses equipment your IT team does not control. A VPN closes that gap.
Networks that block services. Some networks — of a country, a company, a campus — prevent you reaching certain sites. A VPN can restore access, though not always: whoever blocks also adapts.
What a VPN cannot do
This is where marketing deserves scepticism. A VPN does not make you anonymous. If you sign in to your email or a social network, that service knows exactly who you are, because you just identified yourself with a username and password.
It also does not protect you from malware: it is not antivirus. Nor does it clear browser tracking: cookies and fingerprinting keep working the same.
And it guarantees access to nothing. Any service can block VPN addresses whenever it suits them, and many do.
If a provider promises you total invisibility, either they do not understand their own product or they are counting on you not understanding it. We go into more detail in what is a VPN.
So, is it worth it?
It depends what you expect. If you want to encrypt your connection on other people's networks and reduce what your carrier can observe, then yes, considerably.
If you want to disappear from the internet, no VPN will give you that, and whoever sells you that idea is misleading you.