Banded Planks are used in the construction of Water Tanks, similar to the configuration of a Coke Oven. When the banded planks are arranged in the correct order, a water tank will be constructed.
Other than this use, the block doesn't have much use, though it should not be used as a decoration because it may cause lag in a server or single player world, as it regularly checks to see if it forms a complete water tank, similar to a Coke Oven.
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