Question

Why potentiometer is better device than voltmeter for measuring potential difference

Answer 1

A voltmeter should not be used to measure the emf of a cell as a voltmeter draws some current from the cell during measurement. When voltmeter is connected across the cell it amounts to closed circuit measurements and there is inner potential drop due to internal resistance of the cell. Therefore, the measurement of cell's emf will not be accurate
To measure a cell's emf, use of a potentiometer is preferred since in potentiometer-measurement no current flows through the cell. It is an open circuit measurements. The emf of cell will be measured accurately.
Other advantages of using a potentiometer are
Sensitivity of potentiometer is high as compared to that of voltmeter.
The resistance of potentiometer becomes infinite at the time of measurement. Whereas in case of voltmeter the resistance is very high but still a measurable number.