Depending on the severity of the offence, plagiarism can have serious consequences. If you plagiarize, you may receive a reduced or failing grade, be placed on probation, or even be suspended or expelled from the University. Additionally, the offence may go on your permanent transcript, so any future schools or employers see it. Finally, if you are caught plagiarizing, you will lose the respect and trust of your colleagues.
Besides, it's just not nice -- you wouldn't like it if somebody used your work and didn't credit you, would you?