Writing and researching is hard work, and some students are too lazy to do it. Others intend to do the work themselves, but do not manage their time well and find themselves at the end of the quarter with a long paper due and no work to turn in. Some of these students put off doing the work because they don't know how to do it, but wait so long that there is no time to ask for help. Other students think that their English is not good enough todo the assignment and they look for a way to avoid getting a bad grade. It doesn't really matter where the paper comes from.
Some students download a paper from the internet.
Some get a paper from a friend who took the course before. This is the worst type of plagiarism, because when you do this, you are committing a type of fraud that undermines the entire educational system. You didn't learn the material. Your writing skills didn't improve. Your grade is based on deception.