The Plague of Plagiarism
Answer: There are several reasons for mentioning the source. First of all there is a moral responsibility for giving credit since plagiarism is really a subtle form of deceit. Secondly, you will gain as much respect from your audience when you quote something worth hearing as if you were the originator.
Finally there is a very pragmatic reason for avoiding flagrant plagiarism. Joe Biden is only vice-president of the US today because a number of years ago he was dropped as a possible Democratic Party candidate for president after being caught plagiarizing a speech in the Senate.
So watch out because you may also get caught!