Minecraft wiki entry for Time-saving_tips, associated text: The Silk Touch enchantment is surprisingly useful when mining. If you mine, for example, redstone ore or lapis lazuli ore with a Fortune-enchanted pick, you will end up with several pieces of redstone or lapis lazuli for every block you mine. If you mine with Silk Touch, you mine the entire block, and thus it takes up much less inventory space. This allows you to do a lot more mining before you need to return to base and unload. Back at base, you can then use your best 'Fortune' pick to maximize yield from the blocks collected. This decreases the risk of losing the enchanted tool, as well as increasing the time you can spend mining.
Silk Touch is a huge timesaver when building anything large or even moderately-sized out of regular stone or even stone bricks. This is because a pickaxe with Silk Touch will pick up the stone, instead of converting it into cobblestone. This saves all of the coal and smelting time involved in converting cobblestone into stone, letting you use the stone to build or craft into stone bricks almost immediately.