This is done to eliminate the quick black screen you get when switching between SDR and HDR. It keeps your tv and it’s settings as they would be in HDR mode all the time and simply tone maps when SDR is present. (I could be explaining that wrong and invite correction)
Personally, I’m not a fan of this approach and it’s something that really annoyed people when Apple TV 4K came out. My settings are very different on my television between the two modes. I’m sure some may find no issue with this at all, but I thought it deserved a bit of attention. I’m not saying it should be removed, but rather a content dependent toggle should be added, if it’s not there already. At this time I’ve not seen anything mentioned about it being there.