[pizza]

Basically your eyes can see much more than SDR. There are lots of colors that are smooshed together on an SDR monitor that would be distinct on an HDR monitor. Sometimes this means more vivid colors, or sometimes this means much more subtle gradients, or sometimes darker darks that don't get killed by neighboring brights.

Whether the effect is significant depends on a lot of stuff; how the display is built, how the codec works, your viewing conditions, and not least of all, whether the content "looks better" artistically with that much more range.

Just like the compression audio effect clips the range of music and how some pieces of music sound more expressive when the range in amplitudes is used well for artistic effect.

[stavros]

But this only tells me that some monitors can show more colors than others. If you made a monitor that can show more colors, does it matter what you call it? SDR or HDR, it can still show more colors than other monitors.

[davrosthedalek]

The idea is that "full red" should always be about the same brightness (user controlled, so "same"). Not "red" on an SDR screen and "iris burning bright red" on a HDR screen. Essentially, you want to extend your color parameter space to give the iris-burning range new names so that SDR and HDR images can both be displayed sensibly.