Trollers create such threads to troll and catch attention of readers and posters. If no one replies to such threads, they won't keep their monologue for too long. It reduces traffics in trolling-threads and also discourages them to create other trolling-threads.
Right, that's the theory. Now in practice the hypertrolls figured out other ways to get the attention, such as creating frivolous flags and fake accusations and amplify them by abusing self-mod/locking. If you oppose such flags you're supposed explain yourself in the referenced thread but you can't because it's locked and there is another thread but it's self-mod. Basically people get drawn into these trolling schemes not because they really want to feed the troll but sometimes just by following good forum practices.