Why should the EMF of the driver cell be greater than that whose EMF is to be determined ?

EMF of the driver cell should be higher than the e.m.f.s of each of the two cells being compared to obtain the balance point.Otherwise, null point or balanced point cannot be obtained.

Emf of driver cell should be greater because as the circuit starts the potential is dropped across the resistor and more emf is needed to drive current in the circuit

Because the max. potential difference across the resistance wire of potentiometer is the emf of driver cell. Thus emfs to be measured will be balanced in between the extreme points of the resistance wire of potentiometer only if their emf is less than the emf of driver cell. Eg. If we try to measure the length of a line segment, for accuracy, the ruler/scale we use must be longer than the length of line segment. Same way, resistance wire of potentiometer acts a scale to measure potential drop. Thus potential drop across that wire and hence emf of driver cell must be more than the emf to be measured.