Way back when people  wrote letters without access to Wite Out, erasers, or the Delete key, a postscript was a handy way to add a piece of news or a tidbit at the bottom of the letter without rewriting the whole thing. 

The origins of the Latin name  post scriptum, meaning "written after," explain it all. The P.S. comes at the end.

Fast forward to today when the postscript has morphed into more than an afterthought. It is a full-blown,  persuasive writing tool you can use to add information or emphasize your point.