Do you need a VPN on your phone?

You need a VPN on your phone when you use networks you do not control: hotel, airport, campus, office and café Wi-Fi, where the operator and other users can see where your traffic goes. On your own home network and on mobile data, a VPN still hides your address from websites and your destinations from your ISP or carrier, but the risk it removes is smaller, so it is a preference rather than a need. Most people are somewhere in between, and the honest answer is that it depends on how often your phone joins a network you would not trust with your browsing history.

When the answer is yes

Public and shared Wi-Fi. The network operator can see every site your phone contacts, and on a badly configured network so can other people on it. HTTPS hides the contents of most pages but not the names of the sites. A VPN turns everything into one encrypted stream and leaves the network nothing to read. The public Wi-Fi risks post is the plain version of what can actually go wrong there.

Networks that watch or filter. Some campus and office networks log destinations by policy. Some hotel networks filter ports and block common VPN protocols outright. On those, a VPN protects your privacy only if it can connect at all, which is why the app you choose should try several connection methods rather than one.

Keeping your address out of every site's logs. Every website records the address of every visitor. If you would rather those records show a shared server address than your home or carrier address, a VPN is the only practical way to do that on a phone.

When it matters less

At home on your own Wi-Fi. Nobody else is on the network, and the only party watching is your ISP. A VPN moves that view from the ISP to the VPN provider, which is a trade rather than a clear win, and depends on which of the two you trust more with the list of sites you visit. Should you use a VPN at home works through it.

On mobile data. The link between your phone and the cell tower is already encrypted, and nobody shares it with you. Your carrier still sees your destinations, so the same trade applies as at home. Using a VPN on mobile data covers what changes.

What a VPN on your phone will not do

It will not stop malware, phishing pages or a weak password. It will not hide you from a service you are signed in to, or stop apps reading identifiers from the phone. It will not make a slow connection faster. A phone VPN is a privacy tool for the network path, and if you were hoping it was a security tool for the phone itself, the answer to the title question is that you need something else.

What it costs you

Two things, both small on a modern phone. Encrypting every packet and keeping the tunnel alive costs a few percent of battery over a day, more if the server is far away or the connection flaps between Wi-Fi and mobile data. And routing through a server adds latency: a few milliseconds to a nearby city, tens of milliseconds to a distant one. The battery post has the measurements worth knowing.

A quick way to decide

If you do install one, three settings matter

Android's Always-on VPN keeps the tunnel up across reboots and network changes, so you do not have to remember. Its companion setting, "Block connections without VPN", is the phone's real kill switch: it refuses to send anything outside the tunnel, which matters in the gap while a tunnel reconnects. And split tunnelling lets you keep specific apps, banking apps most often, outside the VPN when they object to a foreign address. An app that exposes those three and gets out of the way is doing the job.

Culvert VPN is free with no account or email on servers in the Netherlands, France and Germany, tries several connection methods until one works on a restrictive network, and is on Google Play.