It is a bad thing. Don't do it. The lightest punishment for plagiarism of this sort would be a grade of zero for the paper. Other common punishments are failing the course or even expulsion from school. As you can see, plagiarism is a very serious offense in academia.
Plagiarism is wrong for several reasons.
First, it is lying . If you have been asked to write something as evidence that you have grasped the materials of the course you are taking, offering someone else's work as evidence is a lie. It is no different from having someone else take an examination in your name. Second, it is an insult to your fellow students. When you plagiarize, just as when you cheat on an exam, you treat unfairly those who play by the rules.