Set line length for reading, not for filling

Why this rule exists

A line of text that runs the full width of a wide screen is quietly exhausting to read, and most people cannot say why. The eye has to travel a long way to the end and then find its way back to the start of the next line, and past a certain width it starts losing its place, rereading, or giving up. Centuries of typographic practice converge on a comfortable range, roughly the width that fits a middling number of characters, because reading is a physical act and the eye has limits. Filling the available space is a layout instinct, not a reading one, and the two are often at odds. A column of text is not a container to be filled to its edges; it is a path the eye walks, and the path should be a comfortable length whether or not that leaves the container looking full.