Disclaimer: I am not J. K. Rowling. I do not own Harry Potter.
Professor Cuthbert Binns (deceased) was a ghost, and taught the most dangerous subject of all at Hogwarts – history of magic. As a man who had spent a lifetime (and longer, after death) studying history, Professor Binns was more than aware of just how perilous the subject was, magical history being littered with tales of obscenely powerful artefacts, sanity-shatteringly powerful spells, and all manner of dark lords (officially recognised as evil or otherwise) all of which could lead the average young witch or wizard down the path to destruction, either of themselves or of others.
Professor Binns, therefore, regarded it as a moral duty to put his charges to sleep. The more boring and pointless his charges considered his subject, the safer all round. Professor Binns considered a lesson where his entire class ended up asleep a runaway success – and besides, that gave him extra time to catch up on marking, or writing columns (under pseudonym) for The Quibbler.
Had Professor Binns needed an example to cite to justify his position, he could point to that the last student whom his lectures had failed to induce an appropriately somnolent state in on a regular basis had been one Tom Marvolo Riddle and look how that had ended up…
Author Notes:
May or may not be alternate universe. Just a short scribble based on the notion that maybe Cuthbert Binns means to put his classes to sleep, and could (to his mind) have a good reason for it.