Partially this is so that I can snug everything down and prevent the pin/bolt from vibrating and rotating. With a lock washer the bolt is nice and tight, no movement or rotation to wear anything. I also feel better that the bolt has washers and shoulders for a little more resistance to deflecting or the receiver trying to spread or twist. I just don't like that hitch pins can move around so much.
How much weight your vehicle can handle while it is towing a car or trailer is perhaps the most important factor that people will look at when they are deciding what type of truck hitch to purchase. As briefly stated above, shackle hitches are the most affordable way to get a hitch that can handle weight over 10,000 pounds. Since this is more than the normal weight that people will drag around, many people with heavy-duty needs will get this design.
Their yields are 92 ksi and 130 ksi, so a hitch pin would be 27,600 lbf and 39,000 lbf before leaving their elastic region. Where the difference in grade 5 and 8 comes in is that grade 5 has min tensile yield of 120 ksi and grade 8 150 ksi, so each could potentially fail at 40,000 lbf or 50,000 lbf respectively, so obviously there is a broader region where a grade 5 bolt would bend (from 92 to 120 ksi) compared to a grade 8 (from 130 to 150 ksi). But notice that the grade 5 bolt fails before the grade 8 even hits its elastic limit.
I honestly didn't have too high of expectations when purchasing this product as the cost was a lot easier on the wallet than local farm supply stores. That being said, this thing has a beautiful fit and finish. The ability to have the shackle mounted sideways, keeps from damaging your rope from pinch points. This thing is a solid chunk of steel. I'm not entirely sure that it is possible to damage this hitch on a full size truck. The hitch pin or shackle will fail way before the receiver will. Make sure you use this thing as it is intended because if you manage to rip this off, it is going to take armor plating to stop this thing in a projectile state. It is SOLID! Also, buy a locking hitch pin. Someone would definitely steal this if they have the chance.