I don’t think the purpose of this terrorism is to spread terror for the sake of terror. It’s purpose to provoke a war. Those who believe in jihad want that war to happen but they know that they are only a tiny, tiny fraction of Muslims. Their purpose is to do such horrible things that angry and frightened people will engage in massive reprisals against mainstream believers in the Muslim faith which reprisals, they hope, will eventually spark their hoped-for war.