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Why was antimony added to type metal used in printing?

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A labeled periodic table entry for antimony shows its symbol Sb, atomic number 51, atomic weight 121.76, electron configuration, and name. It is a solid, weakly acidic other metal with a rhombohedral crystal structure at room temperature. — antimony Properties of antimony.

Antimony was added to type metal—an alloy typically consisting of lead, antimony, and tin—primarily for two reasons related to the casting process and the durability of the final product:

Expansion upon solidification: Antimony alloys possess the rare characteristic of expanding as they solidify. This expansion forces the molten metal to fill the small, intricate crevices of casting molds, ensuring the type characters are cast with sharp, precise definition.1
Increased hardness: The addition of antimony significantly increases the hardness and strength of the lead-based alloy, which allows the type to withstand the pressure and wear of the printing process.1