ISO Invariance
Now days, most newer cameras are ISO Invariant, you can shoot one photo at ISO 100 and one other at ISO 1600 and you can take that ISO 100 image and brighten it up to match the luminance level of the ISO 1600 image with no extra penalty in noise.
Photographic dynamic range versus ISO setting with the shutter in single shot mode, silent shutter turned off. (notice the dual gain future at ISO 640 of Sony's A7rII sensor).
Dynamice Range VS ISO
So what is the benefit, what is the whole point?. The idea is that if you take an extreme contrast night scene with bright signs, shoot raw at a low ISO (get a darker image) and then selectively brighten the shadows as needed, which retains full detail in the bright parts without blowing up your highlights, you end up with more dynamic range. This cannot happen when you use the camera to brighten the entire scene all at once.
Capture one readout
As for having dark EVF and dark previews on the LCD, there are hacks you can try. Set the camera to neutral jpeg's with levels cranked up to boost shadows. You get a brighter preview without affecting the raw file.

Sabaton Live at Piraeus 112 Academy - Athens, Greece - 08 March 2017

Sabaton live in Athens

ISO 125 | 1/320 s | f/2.5

The exposure now is been determined by your aperture, shutter speed and the "film speed" as we used to called it is almost irrelevant cause you can simple gain in post.

Aperture: f/2.5 - Could be possible to open up f/2 with a 25mm lens, but wanted to get the most details at the center of the frame, while having a more structured background and keeping that nice separation.

Shutter Speed: 1/320s - Plenty of light to go faster, but a little of blur adds to the movement of the hair, as it gives a sense of motion.