Because Bestigors are really, really expensive. Seriously, Eighth Edition made big blocks of infantry awesome, and that's been a lot of fun, but the cost of armies has gone way up along at the same time. A block of thirty Bestigors is about $120 U.S. at retail. I use a couple of these "herdstones" in each unit to fill the unit out, without breaking the bank.
The stones themselves were easy to make. I took some oven-bake clay from the local craft store, and molded the shape out. After they baked, I painted them in stone colors, and then put a little dab of bloody rune on each one. They're on fifty millimeter bases, so each represents four Bestigors in a unit. I space them out in the second-fourth ranks, and they fit in really well.
I will most likely do something like this for all future armies, like flagellants dragging a shrine, etc. It saves me some money, and adds a little something extra to the army, so everyone wins!