Room of Requirement (spoilers)
Malfoy needed to use the Room of Requirement in order to hide/maintain the Vanishing Cabinet, which was connected to the other one in Borgin And Burkes.
I believe that the book(s) explained in better detail than the films that the ROR is more or less a storage place for things, when it is not needed for some other individual fashion. This is most likely why Harry and Ginny saw it in there when they went to stash the Half-Blood Prince's spell book, as theirs was a similar yet far less sinister need.
We know that Dumbledore was fully aware of the Malfoy plot against him, and asked Snape to do the deed instead. The question I have is was Dumbledore also aware that the ROR was being used in such a way to smuggle in Death Eaters? Seems like a huge security breach if not, and why would he allow possible harm to the students, teachers and castle once he was finished off?