Climbers often carry their water in two ways – using a water bottle, such as a Nalgene, or inside a water bladder, such as a CamelBak.
There are several benefits of using a water bladder. 1) high capacity: the bladders come in various sizes which allow you to carry a large supply of water. 2) hands-free operation: all you need to do is bite down on the valve. 3) encourages consistent hydration: there is no need to stop to drink, so using a bladder increases performance because climbers consume water by consistent sipping rather than gulping periodically.
There are drawbacks to using a bladder as well. 1) inconvenient to refill: refilling the bladder requires removal of your backpack and at least partially taking out the bladder. 2) durability: bladders can leak. 3) difficulty to monitor intake/supply: because the water is out of sight, it is hard to know how much water you have consumed and how much is left, 4) hard to clean: cleaning the bladder is cumbersome. If water is left inside, mold can develop. 5) taste: bladders can make water taste like plastic.