Why did early postage stamp users need scissors or knives to separate stamps?

Early postage stamps were printed on sheets of paper that lacked any built-in mechanism for separation. Because these sheets were imperforate, users were required to use scissors or knives to manually cut and secure individual stamps for use.1

This process changed between 1848 and 1854, when the British Post Office adopted machinery to add perforations—the tiny holes between stamps—which allowed them to be separated easily by hand.1