In my opinion, there are two reasons for this.
- First, there is the issue of being honest about what is your work and what is someone else's. Teachers assign work in order to determine what capabilities the students have, not to determine how well they can copy the work of others. If a student does not give credit to their sources, the teacher has a harder time knowing what work is the student's own.
- Second, there is the issue of the moral rights of the original author. When a person comes up with academic ideas, for example, those ideas are theirs. To take their ideas and use them as one's own is essentially the same thing as stealing -- it is taking someone else's work without giving them credit.
You can use MLA formatting or in works cited. Do not copy and paste anything and if you do, use quotations. If you are trying to pass somebody else's assignment as yours, don't. It won't get you anywhere.