The purpose of providing an iron core in a transformer is to

The purpose of providing an iron core in a transformer is to decrease the reluctance of the magnetic path.

The iron core is used to improve the efficiency of the transformer by providing a low reluctance path for the magnetic flux. This allows the transformer to transfer electrical energy more effectively between its windings.

Key Points:
Iron Core: Reduces the reluctance (magnetic resistance) of the magnetic path, enhancing the transformer's efficiency.
Reluctance: The resistance to the flow of magnetic flux. By decreasing reluctance, the core helps in better flux linkage between the primary and secondary windings.