The simple answer is...keepers have found that their results are much better when the turtles are given calcium supplementation. Hatchlings growth is smooth and even, and females lay more frequently and hatch rates are higher. It is simply results based.
As far as why...think of a cricket. I'll make it local for you. In most of Texas, the earth is loaded with limestone. Limestone contains a very high level of calcium. You may say that a turtle might accidentally eat dirt and get calcium. However, the vegetation that grows in your area typically requires high levels of calcium. Certain grasses are loaded with it. Crickets primarily eat vegetation in the wild. The crickets are gut loaded with high calcium vegetation and the turtles eats the crickets...boom, calcium supplementation. A wild turtle's dietary calcium most likely doesn't come from bones (Incidentally, bones are high in phosphorus. This is why you put bone meal in with your Tulips). It probably comes from guts, especially in insectivorous species like are found in West Texas.
In captivity, people usually feed crickets that are fed on potatoes or are starving. Or, they don't feed crickets at all and feed some other high protein, low calcium food.
I offer calcium supplementation because it is much easier than trying to duplicate a wild box turtle's diet...it's that simple. Even if you could duplicate it logistically, it has been very difficult for researchers to pinpoint the primary diet of a wild box turtle. So, you wouldn't know exactly what to feed and in what proportion. Much of the information regarding diet requirements for "hard to research" animals like box turtles historically comes anecdotally from private keepers and zoos. (Anyone ever try to do a stomach lavage on a box turtle? 
)
More important than a lot of calcium is the diet's calcium to phosphorus ratio, and the captive feeding regimen hasn't quite caught up with this concept yet. The rule of thumb is a 2:1 calcium to phosphorus ratio. The only way this can be accomplished with many food items is to dust with calcium. Interestingly, that ratio is also results based. And, it was originally applied to Iguanids and tortoises(vegetarians). Since I've been following that, though, my results have been incredibly better. If something works, I generally do it.
This post is getting long, but please indulge me on this short story...
When I started reading that captive box turtles should be fed bitter greens and high calcium vegetables, I snorted, "HMPH, yeah right." I had been keeping box turtles for over a decade, and "knew" that a box turtle would never willingly chow on those items. I dusted insects with vitamin/mineral powder, but the majority of nonliving food items I offered were low calcium to phosphorus foods (mostly fruit and tomatoes). My hatchlings' growth was odd compared to wild turtles, my females only laid one clutch of soft eggs(often breaking them while laying), and my hatch rate was only about 50%.
Then, I watched a nature program and saw a video of what appeared to be a wild Gulf Coast box turtle grazing on bermuda grass. The turtle was scarfing it up like candy, and I couldn't believe my eyes! Well, the rest is history. I spoke with other keepers that I respected and got tips on how to enhance calcium to phosphorus ratios. I still occasionally offer items that are not the optimum, but the huge majority of food items are either naturally or artificially 2:1. The difference in results has been huge.
I apologize that this is so long...thanks for indulging me.
