It’s a requirement of international law.  A ship, which engages in international trade, must have a nationality without which it would have no protection under international law.

It is impossible to finance, purchase or sell a ship without being registered as registration provides a means to protect proprietary interests such as prima facie evidence of title and existence and priority of mortgages.

Registration provides a legal framework for the ship and its crew and occupants.

“Vessels registration”, is the process by which a ship takes on a national character.

Every “Flag State” maintains a register in which the particulars of merchant vessels possessing the nationality of that State are entered.