The rendering quality slider mostly affects the lighting simulation quality and the correlated number of light bounces in your scene.
If we stop tracing the ray at a certain distance to save performance (with Medium/High, as done on your images to the right), then we miss some geometric information and count these rays as "they hit the sky" - this leads to an actually unrealistic bright image, where the sky illuminates more than it should. For outdoor scenes, this does rarely make a difference, but for scenes without any sky contribution (like your first image), the difference can be very noticeable.
Generally, it's a good idea to use the "ambient brightness" slider if you want artificially brighter indoor environments. The brightness of Medium or High quality is just a coincidence that comes with the shorter ray traversal paths. I think it's easy to understand if you look at your first image: An occluded scene without windows or light should become pretty dark if correctly simulated.