For holding oil or perfumery (Mark 14:3). It was of the form of a flask or bottle. The Hebrew word (pak) used for it is more appropriately rendered "vial" in 1 Samuel 10:1, and should also be so rendered in 2 Kings 9:1, where alone else it occurs.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Box'". "Easton's Bible Dictionary".