Resetting NVRAM

I tried to reset NVRAM after reading forums that their MB Pro 16 inch stopped detecting their external monitor which was suggested in this link: https://support.apple.com/en-gb/HT204063

After following instructions holding the keys I was told I should hold, my mac went on to reinstall the MacOS instead. What do you think happened?

It's such a bummer as it meant having to deal with all my apps and plugins again. I use my mac for music production so there it goes ...