Answer: Not only does plagiarizing affect the original author/speaker---it also harms your own credibility.
Step-by-step explanation: The original creator deserves credit for their work. But if people are to take it and use it as their own, soon their ownership is lost. And if you are the one stealing (because that's what plagiarism is, a crime), your own credibility is in question. You aren't trustworthy, and you certainly aren't capable of producing your own original work.