Just what is the purpose of exhaust flaps?

Prior to my 2015 BMW Active Tourer none of my cars (Subaru and Mitsubishi) were fitted with the open/close flaps in the exhaust pipes.
The BMW had them and the local indy' BMW garage unplugged them so they were open all the time. No difference in engine performance and exhaust noise just a little throatier.
My new VW T-Roc R has them and when I asked about having them open permanently they said NO, they are linked to engine efficiency. As the car is under warranty for three years I cannot do anything but I just wondered why these flaps are there ?
I know the turbo needs some back pressure but the VW has a PPF and Catalytic converter before the gases reach the flaps so plenty of pressure there.
Noise element is minimal.
So what do they do ?
Cheers and thanks.
Brian.

Thank you, I will search online.
For your information neither the BMW or VW manual mentions the exhaust flaps.
Regards
Brian.

Apologies for resurrecting my old post but I have now had the flaps on the T-Roc R unplugged whilst in the open position. No fault codes and car runs fine.
However ....
I have had this message ....
"The exhaust flaps are a part of the emission control system. On lower rpm the flaps are closed providing some back pressure in the exhaust system. This provides exhaust gas to the EGR valve and lowers the cylinder temperature because there will be less oxygen available. (This also reduces Nox emissions).
This in turn makes it possible to increase the turbo pressure and engine power."
I am not an engineer, just ordinary bloke but I do know that gases are circulated through the EGR valve back into the engine.
No MOT is due until Aug 2025 so I expect I will have to wait until then to see if the open flaps make that much difference.
OR
is the quoted message something to ignore ?

Cheers
Brian.

When in Race mode and the flaps are open the sound is just a little richer, a bit more suited to the car, in my opinion.
I wanted that sound in all modes as I do not want to drive in Race mode all the time.
I could get noise from an expensive replacement exhaust but they are just a bit too harsh for me, I could delete the Resonator but again the YouTube vids indicate a noise that is not what I want. The flap delete is just that little tweak !
Also, and I now may be totally wrong with this idea but I always thought that the quicker the gases escaped from the exhaust the better basically. The VW is only the second car I have owned that had the flaps, all my earlier Japanese and even earlier 70's/80's cars just had the exhaust and later a catalytic converter.
Just making the car suit me, simple as that.