For the same space of a waterskin, I can hold two waterflasks. Waterskin only holds 3.0 liters while two waterflasks hold 4.0 liters. Waterflasks are made from leather, which is easier for a noob to get then a dried cowhide. (Since it involves killing a cow as to any piece of old leather can come from a rabbit skin)
So why do we have them?