After more than six years in higher education and discovering dozens of incidents of plagiarism among my students at various schools, I have realized that the purposes for plagiarism can be categorized four different ways.
1) Laziness. The student waited until the last minute to do the assignment and panics, knowing he won't have time to do it right. So, he pulls an essay or paper from the Internet--and either just adds his name to it or uses parts of it and other online essays--and turns it in as his own work.
2) Too high of expectations. Last year I had a student who came from a long line of writers. Her father was a writer, mother was a writer and grandfather was a writer. She felt her own work never lived up to the "household of writers". When asked to do an essay for my class, she struggled through her own words and feelings of inadequacies and began to interject these words with whole paragraphs or pages she had taken from respected, expert sources, of course without attributing a word of it to them.
3) Doesn't care or thinks the teacher won't know. A partial team of lacrosse players actually said they didn't think I'd catch them after they turned in papers downloaded from the Internet. They didn't care that the work wasn't theirs. They didn't care that they could get thrown out of school for violating the honor code. They just figured the papers were easy to download, the papers were well written, and that I couldn't possibly be familiar with ALL of the papers on the Net so I probably wouldn't catch them.
4) Just doesn't understand what plagiarism is. This happens often with international students. Sure, if you have a school honor code and a plagiarism notation on your syllabus, they probably can quote it to you word-for-word. But, in some cultures, regurgitating words of esteemed experts is considered honorable to the esteemed expert--and is a sign of a well-educated student. These students may not understand that they must document who said the words originally, otherwise it is akin to cheating. As a professor, you want these types of plagiarizers if you are to have any. Once they understand why they must document their sources, they will do so readily. The other three types of plagiarizers aren't as easy to "teach" or change.