People move from one place to another on various purposes. Their stay at one place may not be finite. Hence others need a stable address to which they can send communications.
When some legal necessities arise, the proof of having send communication is needed. At that time if there is proof that communication is sent to the permanent address as given by the person, then that is legally admissible.
Otherwise the addressee can plead as having not received such a communication.
Nowadays they specify communication address specifically.
Permanent address is also needed for background and character checking on some matters. It is needed to verify the antecedents of the person. In this regard it is asked to furnish all the addresses where the person has stayed for a particular period or during a particular period, if the adders furnished is not a permanent address where he has stayed for long period till the threshold date.
In modern days .many times it is not practical to give a permanent address, as we stay in employer given accommodations and go on transfer periodially, and not having a permanent residence till.