Stroke contrast

It comes from the tools. A broad-nib pen held at an angle makes thick strokes in one direction and thin ones in another, and the earliest Latin type copied that directly — which is why old-style faces have a contrast that runs on a diagonal. Later punchcutters worked the contrast up deliberately until, in a Didone like Bodoni, the thin strokes are true hairlines and the axis is dead vertical.

High contrast is spectacular at size and dangerous small. The hairlines are the first thing to disappear — below roughly forty pixels on a screen, or on any paper that absorbs ink — and what is left looks broken rather than fine. Low-contrast faces, where every stroke is nearly the same weight, hold up under almost any abuse, which is why so much interface and signage type has none at all.