When under the effect of a highlevel weakness potion hits that don't deal damage don't recognise as hits (no flashing red of the mob, no knockback etc.)

But if a item with a damageenchant (sharpness/smite/bane) is used to hit the mob, it registers as a hit (mob flashes red, knockback, fire aspect is aflicted, etc.) if the enchantment aplies to the hit mob (sharpness against anything, smite against undead, bane against spiders).

In simple terms: sharpness lets you hit mobs when you shouldn't be able to.