Canteen can store up to 3 uses of water, so if you're filling it with purified water you have 9 points of refill using just one single slot of your inventory, the only other item that can fill 9 point is golden apple juice, so there is no advantage on making juices, they are more difficulty to make, and most of then refill less than the a full canteen of purified water.

If we can fill up the canteen with juice than the story is different, we will be able to refill more points of thirsty using only one single slot of the inventory