By default YouTube chooses quality settings based on the internet connection, but that's not always accurate and often I have to switch from 720p to 1080p because YouTube decided 720p is best.
I'm also aware of some extensions, but most of them allow you to choose a preset quality instead of highest available quality. This means that if I choose 4k as default, when 4k is not available then the extension gets confused and doesn't choose anything.
I'd like to be able to always have the highest possible setting regardless of what YouTube thinks of my connection and which settings are available.