The main reason is to be on the users Home Screen, just one tap away from being on the app. Also, push notifications, pulling people back into the app.
Also there is a security benefit. Phones are protected quite well, so it is safer to keep a user signed on the device and maybe only a PIN or biometrics in front of the app to open it. The apps run in an isolated space, making it even safer.
Lastly there can be a performance benefit. Websites are written in HTML, CSS and JavaScript which is compiled on demand, while apps can be written in languages which are pre-compiled.