In Outlook, one of the obvious use cases for an add-in is to take data from an email and put it in another application. It is then helpful for an add-in to provide an option to continue working in the other application where that data was sent.
On Windows, Mac and Web, add-ins are able to open a browser window to a URL where that data was created in the other application.
On Android and iOS, add-ins can not open a URL in the user's browser. OpenBrowserWindowApi is not supported and window.open creates another dialog within the Outlook app.
This should be a pretty simple addition to the Outlook Android and iOS apps, exposed to the add-in web view via OpenBrowserWindowApi. Example Android code: https://stackoverflow.com/a/2201999/4853706