Not only that... Some memes... particularly some memes with some deeper statements can require extra qualification. And in the PotterHeadsUnite stream there are many discussion about quizzes ruling you to Hogwarts houses, form your patronuses etc, and then a pinned comment can be used to link to the website where the quiz is.... And of course in the case of discussions about very sensitive subjects like religion and politics, a moderator can pin a statement to keep the discussion free of hatespeech etc. etc. etc. All in all, there are many possibilities here.