Emmanuelle couldn't but think about their current situation when that question was asked. The situation being that they were locked out of their common rooms. "If you needed to use a sleeping bag for any reason, like, say, you were locked out of somewhere, you could use the potion to make the sleeping bag larger." And now she was really looking forward to this lesson. Whoever had made the sleeping bags they were currently using had apparently thought 'one size fits all' meant 'make the sleeping bag as small as possible. Not fun when you were an almost graduated seventh year.