Dry bags are one of the most useful things that an outdoor enthusiast can own. They can keep rain from soaking through your backpack and making spare clothes and sleeping bags wet, and ensure that you'll always have dry clothing to change into when you get to camp. They can protect your gear when crossing a river or when canoeing, kayaking, or sailing. At high altitudes (or a humid tropical jungle) they can be used to make sure cameras and other electronic items don't suffer from condensation problems. They can also be used to protect down jackets and sleeping bags and compress them down to the minimum size for easy packing.
While most sleeping bags come with a compression bag or stuff sack, there are replacements and upgrades here if you need them. Just one note of caution: sleeping bag stuff sacks shouldn't be used for long-term storage, especially for down-filled bags. Compression over long periods can damage the insulating power of the down, so when you get home, take the bag out of the sack, give it a good shake, make sure it's 100% dry, and store it inside a generously sized pillow case in a well-ventilated closet.