When you drop the blade all the way, your average 7 1/4' circular saw will cut through two pieces of 3/4' wood stock at a time; so when cutting repetitive shapes like shelves, any DIY'er worth their salt will double the pieces up to halve the amount of cuts they've gotta make.
This little trick is of course used in factory production on materials other than wood, as you'll see in this fascinating Discovery Channel vid of how jeans are made. They take a huge roll of denim -- 1,500 square feet, larger than most of our apartments -- and after cutting it into squares, cut the patterns for 100 pairs of jeans at a time.
The variety of special production machines required to put a simple pair of jeans together is pretty astonishing. And check out the cool vacuum-type thingy that turns them right side out around 4:00.