If the user was told by a smarty-pants computer person to do it, they'll assume it works and is safe. On the other hand, if something goes really wrong, and the user can convince a judge that the mystery binary blob was involved? And the author never said anything about it possibly being unsafe? Then the court can decide that the user had a valid assumption that it would be safe, and the author won't be able to prove otherwise.

They put these disclaimers into licenses because people have already won these kind of cases.