Why emf is measured in open circuit?

Emf is measured in open circuit because it is the potential difference of a source when no current is flowing.

Explanation:

The emf magnitude for any source including battery is the number or value of this open-circuit voltage. Emf cannot be measured in a closed circuit. During charging or discharging of the battery or any source, the emf is not measurable instantly using the external voltage as there is a certain voltage loss inside the source. Emf, according to Faraday is the voltage generated by the force of magnets of the cell.