Why did printers return metal letters to the typecase after printing?

In the context of manual typography, printers returned individual metal letters to the typecase after the printing process was complete so that the characters could be reused for future composition.12

When type is set by hand, a typographer selects individual characters from the compartments of a case and arranges them in a composing stick to form lines of text. Because these metal letters—typically cast from an alloy of lead, antimony, and tin—are durable and represent a significant investment in resources, they are not discarded after a single use. Instead, once the printing run is finished, the form is broken down, and the individual pieces of type are sorted and returned to their respective compartments in the case, making them available for the next job. This process of manual distribution ensures that the printer maintains a functional inventory of characters without needing to constantly cast new type.1345