While the stock is simmering, it is necessary to skim it from time to time. If we don't contain the herbs, we will likely skim them off the top of stock along with the fat and scum. This, in effect, removes flavor from the stock. Most home chefs do not do this, but it is an excellent practice to get in the habit of doing. Basically, it is a question of bacteria.
Stock is a great medium for bacterial growth.
Bacteria love to grow between 40-145 degrees. Chilling stock in ice water lowers the temperature of the stock rapidly. The stock does not then spend much time between 40-145 degrees and this greatly reduces the possibility of bacterial growth. To do this, cover the rinsed bones with cold water. Bring to a boil.