In a Potentiometer why is emf of driver cell( source) taken to be greater than that of the cell whose emf is to be found out????

The source cell produces a potential  difference across the potentiometer wire and this potential difference falls uniformly across the wire.When a cell of emf greater then the emf is used then then it cannot be balanced on the wire as the wire will have a lower potential difference then the cell.

For example suppose the source cell has emf 10 volt then it can generate a  potential difference of 10 volt or even less across the wire and so we can not find balance point for 12 volt on the potentiometer hence cell of emf 12 volt can not be balanced.