Casing is the major structural component of a well. Casing is needed to:
Maintain borehole stability
Prevent contamination of water sands
Isolate water from producing formations
Control well pressures during drilling, production, and workover operations
Casing provides locations for the installation of: Blowout preventers
Wellhead equipment
Production packers
Production tubing

The cost of casing is a major part of the overall well cost, so selection of casing size, grade, connectors, and setting depth is a primary engineering and economic consideration.