I make a lot of stock. I have spent years making a lot of stock. This is what happens when you raise animals and butcher them: There are lots of bones, and therefore, lots of stock.
The big problem with stock, however, is that it takes up a lot of space. If you freeze it, you end up with big quantities of stock in the freezer. If you pressure-can it (my preference), you end up with dozens of jars of stock.
It's just a lot of real estate for meat-water, you know?
Given my continous making of stock, and therefore continous problem with storage of stock, you'd think it wouldn't have taken me literally years to realize that because stock is mostly water, I could reduce the water significantly and make a stock concentrate that would then take up significantly less space.
Years! But better late than never, I guess.
So now whenever I make stock, I continue simmering the stock after I've strained out the solids. I do this for however long it takes to reduce the stock by half, usually at least a few more hours. That way, I only have seven quarts to can or freeze, instead of fourteen.
This makes a very, very concentrated stock. And it is delicious. The water can be added back when you drink it or cook with it if you want, but really, the stock is so much better when it's left pretty strong.
And that's it! Reduce your stock, both for space-saving purposes and much better flavor.
(Still can't believe it took me so long to think of this.)