I agree that 250 nits is too low, but my monitor clocks in at 400 and HDR already looks better, if only thanks to the increased colour channel resolution - particularly visible in highlights, clouds etc. Where there previously was just a single colour blob I now can observe details impossible to display with just eight bits per channel.

Interestingly my laptop's display reaches 500 nits and that is already painfully high outside of midday hours. My phone goes to 875 and I find that only to be useful outside in the summer sun.

The difference is between SDR and HDR. Going full blast with a full image at 500 nits or having an image averaging 200 nits with only peaks at 500 are two vastly différence things.