Yes, leaving a layer of solidified fat on top of stored stock is useful. It acts as a barrier, preventing air and bacteria from reaching the stock and thus extending its shelf life.
When stock cools, the fat naturally rises to the surface and solidifies. This layer of fat creates an airtight seal, protecting the stock from oxidation and contamination. Oxidation can lead to off-flavors and spoilage, while bacteria can cause the stock to become unsafe to consume. By preventing air and bacteria from reaching the stock, the fat layer significantly slows down the degradation process. This method of preservation has been used for centuries, before modern refrigeration, to keep stocks and broths edible for longer periods. The fat effectively blocks the stock from exposure to the elements that would cause it to spoil more quickly. When you're ready to use the stock, simply remove the solidified fat layer. The stock underneath should be fresh and flavorful.
If you prefer a less fatty stock, you can chill the stock completely, remove the solidified fat, and then store the defatted stock. However, remember that removing the fat layer will reduce the stock's shelf life, so consume it sooner.