So (apparently) unlike most people posting so far, I'm a faculty member at a university. Here's why we use Turnitin;
1. Something like 10% of all work submitted for assessment at college (university in the rest of the world) contains plagiarism of one form or another (far too many citations to bother listing)
2. The perception that other students are successfully plagiarising is believed to be a risk factor promoting students to plagiarise (along with a sense that the assessment is meaningless, faculty don't know or care about students as people, and poor time management skills)
3. Gathering the evidence to formally address an instance of plagiarism takes anything up to a couple of days vs a minute with Turnitin
4. The system makes it possible to treat everyone equally. The alternative, which I have seen many times, is that the faculty member pre-judges the student and looks harder for plagiarism when the work is 'too good' for the student. This commonly occurs for non-white middle class students, despite the evidence that white middle class successful students plagiarise to a similar extent that non-white, non-English speaking background students do. Sadly, subconscious racism is very much alive in academia
5. Turnitin does not label a piece of work as plagiarism - the faculty member reviewing the report does. Direct copying is only one of a variety of ways that plagiarism occurs. Turnitin is the only practical way I know of that faculty can use to detect plagiarism by citation - where you steal someone else's bibliography.
For those of you bitching about your loss of copyright - (re)read the decision. The judge is very clear that you have lost nothing and are in no risk of losing anything. Consider this case one of the consequences of the right of "Fair Use" - the rest of the world would love to have the same freedom but US Trade representatives are hell-bent on making sure we don't.
Finally, Turnitin does not show the work to other people, other than your instructor without their permission (which most can't give as its not their work). Most matches turn out to be to public sources or to work of students in the same programme. When we get a request from an outside institution we refuse it- and then immediately re-run the Turnitin report for the student in question.