I am just curious what the small app Windows Insider actually does. The users logs in to their account, selects Slow Ring or Fast Ring and then the user is prompted to confirm that their phone will be configured to receive preview builds. I am curious what kind of configuration this app actually does. Could not it be done in some Windows Settings item without using an app and why?
It could be done as part of the windows settings, but the Insider program did not exist when the settings for Windows Phone 8.x was being written.
By having this as a separate app, this additional functionality, which in turn is only intended for developers and enthusiasts, the option can be brought forward to devices that pre-date the insider program.
Historically, there was a "Preview for Developers" app that allowed advance installation of updates for Windows Phone 8.
I suspect that both of these apps have a flag set in their package that allows the apps to change the update server configuration, so that phones then think that updates are available that are not published on the "normal" servers.