world war one - What was this 1917 shell casing used for

Bought this shell from an antique store. Would like to know the history behind it, as in what it was used for? (The casing is about 16 and 1/2 inches in length)

Original France WWI French “Souvenir de la Guerre - 1917” Austrian Ber – International Military Antiques

18 pdr shell case-markings - Arms and other weapons - The Great War (1914-1918) Forum

German Brass Shell Casing WWI Aug. 1917 Patronenfabrik Karlsruhe 622 SP255

simonjoneshistorian.files.wordpress.com/2014/02/e4

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