Purpose
The purpose of the State Seal is to certify formal documents issued by the Pennsylvania government are official. A seal is applied to a document by embossing the document, or affixing the surface with an impression. Seals were originally pressed into wax or clay.
See this seal from the town of St. Marys used in the 17th century.
https://www.hsmcdigshistory.org/clues-to-early-maryland-21-seals-of-identity/
The original Seal of the Province of Pennsylvania was the Seal of William Penn to represent the proprietary government of the Penn family. See below.
The Pennsylvania legislature uses the Pennsylvania State Seal as their symbol because they are the body that creates and issues documents on behalf of the the Commonwealth.
The current State Seal was formally adopted in 1777. See both sides of current Seal below.
