Known uses Edit
Water skins are advantageous to whoever possesses it, especially in duels or large scale battles, as they provide waterbenders with a portable source of water. When several are used in concurrence, they can provide tremendous amounts of water, making up for any lack of it in a given environment. A downside to the use of a water skin, however, is that they do not contain an endless supply of water, and need to be refilled from time to time. Nevertheless, this can be circumvented by bending the used water back into the skin.
As a prominent user of these pouches, Katara demonstrated on many occasions the importance of a water skin. For instance, while in the Fire Nation occupied stronghold of Omashu, Katara was able to effectively defend herself against Mai and Ty Lee until the latter blocked her chi, due to the supply of water readily available from her pouch.
Also, during Team Avatar's attempt to break into the Earth Kingdom Royal Palace, Katara used water from her water skin to create an ice slope that elevated her, allowing the creation of a much grander water whip with the water from the moat. With only the water from her pouch and the amount available within the vicinity of the palace, Katara was able to defeat several units of Royal Earthbender Guards.
During the Invasion of the Fire Nation, Katara used three larger water pouches to compensate for the lack of water in the Fire Nation Capital. With these pouches, she was able to snap the chains of a ballista, and defend herself against the attacks of Imperial Firebenders, among other things.
The different sizes of the water skins also allow people to carry them underneath their clothing and thus launch a variety of sneak attacks. When Avatar Korra faced three Triple Threat Triad members, the waterbending member was able to suddenly assault her with a water bullet due to the use of a hidden pouch.
In cases where healing is necessary, it is advantageous to have a water skin, as a body of water will not always be present when someone is injured. Water skins allow waterbenders to treat wounds acquired during a battle as soon as they happen, making the healing process more effective.
Lastly, Katara was able to heal a badly injured Zuko after his unsuccessful attempt to redirect a lightning attack generated by Azula. After she defeated the Fire Nation Princess, Katara quickly healed Zuko's wound to bring him from the brink of unconsciousness.
Other purposes Edit
A water skin can be quite useful in other times of need, such as when Sokka and Katara were sick. The latter asked Momo to bring back fresh water at Sokka's request, giving the lemur her water skin to fill since Aang was not present. Momo misunderstood her commands, and thus brought back various things rather than the water needed. The water skin would have been helpful in the scenario had it been any other being fetching the water.
The presence of water skins in arid conditions where there is a limited supply of water can be quite beneficial. While traveling through the harsh Si Wong Desert, for example, Team Avatar was able to survive, albeit barely, with the water available in Katara's water skin.
Water skins can also be useful in missions of sabotage or infiltration, as evidenced when Team Avatar penetrated the Fire Nation drill headed for Ba Sing Se, in an attempt to dismantle the machine by targeting its interior frame. As the drill was mostly metal both externally and internally, the water skin provided Aang and Katara a source from which to bend. Working together, they utilized the water to slice some of the metal beams within the mechanism, weakening its frame.
Later on, Aang used water from the skin to carve a target onto the drill's center-most point, where he later used a combination of airbending and earthbending to accumulate enough momentum to render the drill inoperable.