There are a few different levels to this question. As a student, plagiarism is bad for your development because you are not learning. From a teacher's point of view, plagiarism makes it harder for teachers to help you effectively. There's also the wider level of the academic community as a whole, which sees it as anti-social . Finally, readers would prefer to read honest research. As a result, plagiarism is taken very seriously. Lets look at these points in a bit more detail.
Assignments are designed to help you learn and develop your ideas. Plagiarism does not do that at all. It is like not going on holiday, but showing your friends someone else's holiday photos. You might impress people in the short term, but you get none of the benefits of actually having a real holiday.
Your tutors need to know how to help you. Tutors will analyse your assignments and make a judgment about where you are in the triangle (see first picture, above). They will then know what skills you still need to develop and therefore what type of exercises and advice to give you. They can't do this if they only have plagiarised texts.
Your readers may want to find out more about a point you make, so it is essential to use references. This makes it easy for readers to follow up. Readers will also be sceptical if you don't use references.
If plagiarism is discovered, it can have bad results for both your academic and professional careers. Plagiarism is seen as a type of cheating and so employers are less likely to employ offenders. For the same reason, universities take a dim view of plagiarism. They also want to keep standards high, so preventing plagiarism is a type of quality control for universities.
Academics value such things as: trust, reputation, ability, originality, reliability, honesty, and being committed to finding the truth in your subject. So, plagiarism threatens to undermine that whole system.
Try this interactive activity to see if you have got the main points.