Why use a VPN
People turn a VPN on for a few concrete situations. The same app leaves a lot of problems untouched.
Public Wi-Fi
On a cafe, hotel, or airport network, other people are on the same local network as you. HTTPS already encrypts the contents of most websites between your browser and that site. A VPN puts the rest of the trip, from your device to the VPN server, inside an encrypted tunnel. The shared network sees a connection to the VPN company. It does not see the list of sites inside the tunnel.
Connect the app before you sign in to anything. Traffic you send before that still crosses the open network. A VPN also will not notice a lookalike Wi-Fi name or a fake login page. Read the network name and the site address yourself.
Your internet provider
Without a VPN, the company that sells you internet service can see the names of sites you open and when you open them. With a VPN connected, that company sees a connection to the VPN server and the amount of data, and not the pages inside the tunnel. The VPN company is then the one in that position. You have moved the view from one company to another, which helps when you trust the VPN company more than the network you are on.
Streaming
Streaming services decide what to offer partly from the address you appear to come from. A VPN server in another country presents that server's address, so a catalogue can change. It does not always work. Services block addresses they believe are VPN servers, and a provider's app can fail on a show that worked last month. Treat streaming as a maybe, and use the provider's refund window if a specific show is the reason you subscribed.
Travel
On the road you borrow other people's networks: hotels, airports, a relative's router, sometimes a network that blocks sites you use at home. A VPN gives you one encrypted path back to a server you chose, which can also make a blocked site reachable when the block is based on your local address. It will not get you through a block that also stops VPN servers. Carrying the app already installed, and signed in, saves a fiddly setup on hotel Wi-Fi.
What stays the same
A VPN does not make you anonymous. The VPN company can see that your account is connected. Sites where you have an account still know it is you once you log in. Your browser still carries its usual stored logins.
A VPN is not antivirus software. It does not scan files, catch malicious downloads, or stop you typing a password into a fake page. Those are separate tools and habits. If a provider sells antivirus as an add-on, that add-on is a different product sitting next to the VPN.