Boning is not used in everyday garments. You will find boning in formal wear and gowns, stapless garments and swimwear. Boning helps a garment hold it's shape, or stay upright without sagging. Collar stays, the stiff points in the corner of collars, are a form of boning.
Boning is available in many, many forms and they way it is sewn depends on the type of boning. Boning was originally made from bones. It is now made from plastic, metal or stiff polyester. Boning is commonly sewn to lining or encased in seam allowances or casing. It is not directly sewn to the outer fabric of the garment.
Sewing costumes that may be worn once, does not require you to search out boning or break the bank to purchase boning. Package strapping, those stiff plastic strips wrapped around boxes, or other plastic packaging, vinyl shower walls can all be used to obtain shaping or stiffness. Hated Packaging Stiffens Costume Parts