ISO still matters with RAW because camera ISO is usually applied as analog gain before readout and digitization. That is not the same as brightening later in software.

If you underexpose at base ISO and lift it in post, you also boost read noise and quantization noise introduced during readout/conversion. Raising ISO in camera can improve the signal level before those stages, which often gives a cleaner result in low light.

Also, if the recorded values occupy only a small part of the available tonal range, you may lose usable precision. Brightening later cannot recover detail or tonal resolution that was not captured well in the first place.

That said, at a fixed aperture and shutter speed, ISO does not increase the actual photons hitting the sensor. It changes how strongly that captured signal is amplified and encoded. In normal exposure decisions, ISO works together with shutter speed and aperture: increasing ISO lets you use a faster shutter or smaller aperture for the same brightness target.

So the short version is: RAW gives flexibility, but camera ISO is still part of the capture process, not just a cosmetic brightness slider.