Why ordinary white has a ceiling.
In an sRGB image, the highest red, green and blue values describe white. That does not tell an HDR screen to use all of its available brightness. Ordinary page content is rendered within the standard brightness range of that viewing setup.
HDR content can describe highlights beyond that range. The extra headroom is what makes a white mark stand apart from the surrounding interface on a compatible display.
What PQ adds.
PQ, or Perceptual Quantizer, relates encoded values to luminance. It is one of the transfer functions included in the ITU-R BT.2100 HDR recommendation, which uses BT.2020 color primaries.
Glowhite converts the source pixels and embeds a Rec.2020/PQ ICC color profile in the JPEG. Merely attaching that profile to an unconverted sRGB image would not produce the same result.
Know the format’s limits.
This is a focused tool for bright graphic artwork. Its 8-bit JPEG output can show banding in smooth gradients and compression around edges. It does not recover highlight detail that was never present in the source.
An embedded HDR profile also does not guarantee universal playback. Software may compress the brightness range or interpret the file differently. Inspect the result where it will actually be seen.