Previewing the web app on different device templates provides the web app designer a way to see how the app will be experienced by the end user group. The app may be experienced on many different devices or only a small number. This option is good to check once is awhile.
The “Launch” option opens up the web app “viewer” in a new tab. The “viewer” app is what the end user will experience (without all of the app builder/designer options). The site doesn’t “have to” be launched, but is a convenient way to view the web app from the end user’s point of view.
The video concludes by going back to the MyContent area so you know how to find the “View” option on the web map app viewer in case you need to copy/paste the URL to send to someone else or just be able to view the web viewer as the end user would experience the app.