One gains consistency. Various categories of content have exactly the same colour: light backgrounds and dark text and borders (or vice versa), different hues

One gains consistency. Various categories of content have exactly the same colour: light backgrounds and dark text and borders (or vice versa), different hues (e.g. traffic lights red, yellow and green like in the article) for specific text and box meanings. Nothing should have an ambiguous or arbitrary colour. For example, the HN comment editing page can get away with 3 or 4 gray shades plus some in browser UI elements because they are completely unrelated objects: main background, button background (darker than main background, needs to be just noticeably different), dark grey text (also darker than main background, needs to be legible) and dark grey arrows (not text and not buttons, and therefore not necessarily matching the text and button shades).