The transfer serves to protect the pledgee's real security right by preventing the pledgor from alienating the pledged object or pledging it to some other person.
Personal security rights are regulated by the law of obligations (contract and delict), whereas real security rights are governed by the law of property.
The transfer of possession is intended to publicise the fact that a real security right, in the form of a pledge, has been established over the pledged object.
Unless otherwise agreed by the parties, a real security right is indivisible, securing the entire debt and binding the secured party (or debtor) until it is paid.