Great book that exposes what happens when you donate your body for research: Stiff: The Curious Lives of Human Cadavers. You don’t always end up being used for medical student practice. You could be dropped from a height to test what happens, you could be used for product research, etc. If you don’t want any of these things to happen to your body after death, you need to make a specified donation.
...Or buried for a month or two in a pasture, then dug up and see what happens to a human body during, say, Summer. (Not that it matters after death—you're not feeling a thing, and the forenics folks are generally respectful of the donation).