Instrument - orchestra: Why pipe organs need a mirror?

Yesterday evening, I was able to watch the program on Lithuanian national TV "Stories of Things", and from 35 minute mark the show was about the pipe organ.
On the screen you will see Professor Leopoldas Digrys and Renata Marcinkutė, organ restorer Rimantas Gucas and you can listen to pipe organs of Vilnius St Casimir's and VU St John's churches. At the end I was able to get on screen as well with @laputis when we practiced our organ duet. Our assistants were Audra Telksnytė and Eglė Rudokaitė.

​https://www.lrt.lt/mediateka/irasas/2000081012/instrumentas-orkestras-kodel-vargonams-reikia-veidrodzio?jwsource=cl