Many extensions support multiple installation types (panel, overlay, component).
A Streamer must select where to install it then.

Some Streamers preferr the overlay mode but some viewers don't like overlays, so then got the option to disable it on twitch.

Now the viewer does not have access to the content in the extension anymore.

If Twitch would allow streamers to simultaneously install such extensions as overlay but also as panel, the viewer would not be forced to allow the unwanted overlay if he/she wants to use the extension's features.