There are several reasons you might want to use external sources in your paper. Just like you, your paper wants to get out every once in a while. It's been cooped up on that page all day. Have some mercy.
Maybe it's a requirement of the assignment, or maybe you find an external source that helps you build your own argument. Maybe you are writing your own argument as a dialogue between several ideas from several writers, or maybe your big brother is begging you to quote the university paper for which he is the editor-in-chief. (The article on overpriced cafeteria food isn't really relevant to your paper on global warming, but you can't stand to see that look of disappointment on his face.)
Whatever the reason might be, external source material has the potential to make your essay even more interesting (and convincing) by bringing in the views of experts and thinkers. (Surprisingly, Rodin's sculpture doesn't have all that much to say. For all that thinking he's always doing, he's likely to decline your interview.)
Try to ensure that your external sources come from a trusted source. As in, more trusted than TMZ. Many educators do not consider Wikipedia to be a respectable source, so it's always a good idea to check with your teacher if you're unsure about the quality of your source. Make sure you use quotations the right way, and don't forget to cite. So many students muck this up that the use of proper notation can often be a cite for sore eyes.