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I just use one of these so I can use any water bottle if necessary. I have one with a wide mouth adapter so I can use one of those metal bottles that won't make the water taste like shit.
I can't think of any scenario where it would be absolutely necessary that it would require it instead of just grabbing a water bottle from my backpack, but since I do wildlife photography it removes the inconvenience of having to stop and put down equipment to use both hands, just take a sip and keep moving, and since it uses a water bottle instead of larping bladder, it's very easy to refill.
