Good point, Bruise.
Bruise has hit on it -- Before metal armor, weapon edges were flat ground or almost hollow ground. With the advent of metal armor, edges tended to be either convex ground or saber ground for edge retention. The function of the weapon went from slice to shear.
This edge geometry shift happened in europe just after the migration age, and the shift also occurred in Asia around the feudal periods (14th to 15th c.?). Before that time, the protection used by combatants was little more than skins and furs and a shield (bamboo, and leather, or heavy cotton cloth in Asia), and the flatter grinds prevailed against these softer armors.
These days, the blade geometry is still determined by the use of the cutting tool. When a really sharp edge is called for, but the material being cut is relatively soft and cutting done withouut impact, hollow or flat grinds are used. (i.e, straight razors, woodworking and leather tools, light duty chisels or gouges, craft knives). When the edge doesn't have to be so finely honed but it needs to stand up to impacts and abuse, you more commonly see the convex or saber grind (some Axes, adzes, heavy use chisels, blades meant to cut metals or stone/sheet rock).
