Leaving aside possible legal consequences for copyright violation, one of the worst effects of plagiarism in everyday school and college assignments is self-deception, kidding oneself into thinking one has a better grasp of the subject matter than is the case. Then there is deception of teachers, which is shabby and dishonest. In many schools and colleges there are various "pains and penalties" for plagiarism.
Simply copying what someone else has said does not provide any insight into the subject. Examining more than one viewpoint, and how they were formulated, avoids making factual assumptions and in many cases mistaken inferences.