The Jedi Order was an ancient, monastic peacekeeping organization united in their observance of the Force, specifically the light side. A noble order of protectors...
Anti-possession symbols are a method to ward off demonic possession. It features a pentagram, or endless knot, which is the reason why it protects those...
They make Wrackspurts (invisible creatures that floated through one's ears causing their brain to go fuzzy) visible to the wizarding eye, well, according to Luna Lovegood.