What was the purpose of postage stamps?

A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail). Postage stamps have facilitated the delivery of mail since the 1840s. Before then, ink and hand-stamps (hence the word 'stamp'), usually made from wood or cork, were often used to frank the mail and confirm the payment of postage.

Source: Wikipedia