Powers are given by the Constitution or General Assembly. Counties do not have home-rule. Counties only do what is expressly authorized by the State. The primary purpose for counties is to perform State functions at the local level.
Counties can buy and hold property, sue and be sued, enter into contracts, etc.
They have received broad powers from the State over time, and now deal with a broad range of services. Any three justices together constituted a quorum for conducting business as court of Pleas and Quarter Sessions. Court had some judicial duties, such as civil cases, probate, guardianships, administration of estates, and minor criminal cases. All Boards had five members elected at-large by voters of the County. The Township Board carried out County Board policies on roads and bridges and assessed property for county taxation.