Hello,
During the lifetime of your grandfather the Power of Attorney will remain effective. That means your father can dispose of the property on the strength of the power of attorney. However, your grandfather has the right to revoke the Power of Attorney.
On the other hand, after the death of your grandfather, the power of attorney will become redundant. And the Will will come into force. Then your father can only dispose of the portion which had been willed to him.
So the POA is the document which remains in force during the lifetime and Will is the document which comes into force after the death.
Regards,
Nishant