To have a copy of your emails on your device, for backup, and offline use.

A web UI can do that

The comment I was replying to was wondering why people installed apps to read email instead of using the browser based clients. So I don't understand your point ...?

You can have a copy of your emails for offline use in your browser.

Less so as a reliable backup.

To be clear, a "Web UI" usually refers to a desktop or mobile application whose user-interface is built with web technology (HTML, CSS, JS etc.) that is rendered using a webview component in the application. You don't refer to a website (even if it is a web-based application like GMail) as "Web UI". It is still called a website. And while you are right that browsers today can store data for offline use, nobody considers it good enough for a backup.

A web UI cannot do that. It is simply not possible.

Websites can work offline

Because native apps are faster, work when you aren't online, and are all around more pleasant to use.

> work when you aren't online,

...but why do you need that?

Why does it need to be that fast too?

I guess many more people here are real power-users of email than I would expect for a forum full of engineers lol