Wikipedia says:
A postscript may be a sentence, a paragraph, or occasionally many paragraphs added to, often hastily and incidentally, after the signature of a letter or (sometimes) the main body of an essay or book.
When all letters were handwritten, and adding a new thought to the letter would have likely involved rewriting the entire letter, a postscript had obvious practicality. Now, however, one can just as easily add the thought to the main text.