As far as I can tell, to save Sirius' ungrateful hide.

Miss Granger, Miss Parkinson, Mr Marvolo, the young Mr Malfoy -- It is IMPERATIVE that you sedate them as soon as possible, for as long as possible with a single dose. MLE is making noises about Veritaserum. Come up with some reason - any reason. We shall say they were Obliviated. It will be enough. They will agree to it. They must. Explain it to them if you need to.