What is the point of setting an upper limit in Auto-ISO?

Perhaps, but you asked why people do something, and I gave some reasons why they might do it, within the context of their habits and logic.

Thank you for remembering that qualifying statement, "if exposure is held constant". Some are not qualifying, and that only causes confusion.

Most beginners are NOT shooting in M mode, and when they hear "turning up the ISO", they are thinking of what happens in those modes where the exposure time and/or the f-number are automated.

Thus the persistent and erroneous belief that high ISO causes image noise.

in certain camera models after certain ISO a non switchable NR might kick in for raw data... which may or may not be detrimental for one's purposes... granted it is rarely around a nominal ISO6400

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Ah!

Thanks, Jim!

In M mode, with fixed exposure (aperture and shutter speed setting), changing ISO adjusts only the image's brightness and has minimal effect on noise.
In an automatic mode, ISO indicates whether the exposure is low or high. In an automatic mode, higher ISOs indicate low exposure, and low exposure has more noise.
If you specify the upper Auto-ISO limit in A mode, the camera will attempt to keep the exposure and noise the same by lowering the shutter speed once the ISO limit is reached. Of course, this works only if you have a minimal Auto-ISO shutter speed higher than necessary.

Yes, and thank you for the answers. However, the intent of my question was, why are you using the upper bound in Auto-ISO?

It seems that some use the bound to stop the increase in noise. This works in automatic mode if we allow for the shutter speed to go below the minimal setting. However, one should consider that it is easier to remedy noise than motion blur.

I prefer using the displayed ISO value rather than EVF brightness to judge how weak the exposure is. However, some cameras do not show the actual ISO value until the shutter is half-pressed (Auto-ISO mode).

yes we agree, i posted some images and got shot down for doing so. lower exposure and iso introduced WB shift as well blotchy shadow noise, if the subject was red even worst so shadowed skin tones would suffer.

That can happen with some cameras and some raw developers at aggressive pushes. It can usually be fixed with the calibration panel in Lr, but I prefer to avoid the really aggressive pushes.

Here you need to differentiate between amount of noise and visible noise.

In the scenario you describe the amount of noise will be the same, because the exposure* is the same, but in ISO 6400 shot more of that noise will be visible.

* exposure - amount of light that struck the sensor per unit area while the shutter was open.

I assume that both shots will be brought to the same brightness in the post. In this realistic situation, the noise visibility will not differ.

If shooting raw, why should the noise in the ISO 6400 shot be more visible?

Because the ISO 6400 is brighter initially and before any adjustments, a purely theoretical argument.

I don't think how much noise there is with default development settings is relevant.

For sure that’s unusable! Duh! Your shutter speed was so slow that the animal died and rotted before the exposure finished.

In sincerity, the white balance looks off to me. I’d need to look at it very critically to know if it would be suitable for putting on the museum’s website or annual report. I’d generally say it’s not quite usable for such a purpose.

Have you seen actual colors or you just have a gut feeling?