Previously, letters were paid for by the receiver unless already prepaid by the sender. As you can imagine, this system was chaotic and hard to enforce, for the receiver could send the mail back to the post office without paying for it. Once stamps were issued, payments were regulated and the mail moved much more efficiently. By 1855, stamps became mandatory.
Words of Wisdom
If you would not be forgotten
As soon as you are dead and rotten,
Either write things worthy reading,
Or do things worth the writing.