Aside from turning in a paper that doesn't follow universal guidelines, you're also denying yourself a portion of education by plagiarizing another person's work. Instead of learning how to correctly write a research paper, look up information and cite sources, and ultimately form your own opinions that relate to whatever it is you're writing about, you're simply taking work that others have put effort into and claiming it as your own. You're skipping important skills that you could very well need in the future, whether in another educational setting or throughout your career. On top of that, you won't receive true feedback about how you've performed while researching and writing your piece. Critiques are integral to learning and expanding your knowledge.
If you're caught plagiarizing, whether you're writing a paper for high school, a college program, or for your job, you'll immediately lower your integrity and credibility. It will be assumed that your performance is weak as well, since you decided to rely on someone else's work to complete yours. Even unintentional plagiarism will be taken seriously and reflect poorly on you.
Not only will plagiarizing lower other people's respect for you, but it's also disrespectful to the people who you are plagiarizing. Depriving an author of credit that she deserves and earned is showcasing that you don't care about her hard work, but only are concerned with the credit that will be given solely to you. Technically, plagiarism is the stealing of another person's thoughts and words.
Plagiarism is not only unfair to you and the original author, but it's not fair to your fellow students or co-workers. Your work will be compared to theirs, and ultimately their work will be graded unfairly in comparison to yours. This is especially true if you plagiarize an excellent source and don't end up getting caught. You're cheating your classmates out of receiving a fair grade based on everyone's own, personal effort and work.