As for what they were designed for, somebody above already noted that what they were originally "designed for" was weeding and breaking up clumps of ground.
I train with long sticks. Some of my moves involve stabbing motions with the long sticks, poking somebody in the face or solar plexus. Somebody could bleed because of it, because of the stabbing motion I made. And yet, I manage to refrain from calling for sticks to gain piercing damage. Just because you see a technique that shows somebody bending over in pain after getting poked in the gut does not mean that the sai should be a piercing weapon.
Heck, you could make the same argument for unarmed attacks getting to become piercing, too, since I use "spear-hand" attacks and key strikes that are designed to strike at soft-tissue points.
I train with long sticks. Some of my moves involve stabbing motions with the long sticks, poking somebody in the face or solar plexus. Somebody could bleed because of it, because of the stabbing motion I made. And yet, I manage to refrain from calling for sticks to gain piercing damage. Just because you see a technique that shows somebody bending over in pain after getting poked in the gut does not mean that the sai should be a piercing weapon.
Heck, you could make the same argument for unarmed attacks getting to become piercing, too, since I use "spear-hand" attacks and key strikes that are designed to strike at soft-tissue points.