You don't have to install any AV on Android. Waste of memory, battery and RAM. Of course Anti-Theft is an another story.
Partially you are right, if you don't download apps or use Google Store, you are reasonably safe, even if lately dozens of malware are also present on the official Store because of lack of controls.
Using your brain and common sense you're also pretty safe, but human error is always possible, and we consider that a smartphone usually contains a lot of personal data, photos, accounts.
Because of that I use Bitdefender free, lightweight and not invasive.
But an antivirus for Android doesn't have automatic modules of behavior and defense as on Windows: generally, these applications are limited to the identification of the threat and then leaving the user the task of neutralising it, by uninstalling the infected app.
The developers, knowing that only the prevention is not sufficient to encourage a user to download their antivirus, add features that make the smartphone more secure: localization of the device in case of loss, formatting by remote, activation of the camera and ringer in case of theft, etc.
These additional features can justify the price of a paid product but personally I don't need of them, that's why I use just a free "second opinion" tool (after my brain).
