In Stars!, an old 4X game (regarded as the first 4X game, in fact, if memory serves), there always appeared "Mystery Trader" ships in your game area some time into the game. These would travel at ridiculous speeds from one edge of the map to the other, sometimes changing speed and/or course slightly on their way. If you managed to intercept one with a fleet carrying at least 5000 kt of minerals (a fairly significant amount that required mid-game technology and resource levels), they gave you a gift in the form of technological advances, artifacts, ships, or blueprints you could use to build ships.
My point is that the artifacts and ships you received were not only powerful and versatile, but always, as a rule, also exotic -- stuff you couldn't build yourself as they weren't even in the game technology tree. After that, strange moods in DF have always been almost comical to me, like some sort of Lord of the Rings parody ("you must melt the One Gold-Studded, Gem-Encrusted Floodgate With Hanging Rings of Bronze and an Image in Leather of Carp in mount Doom, or else Sauron will use it to decorate his dining hall and give all his orcs lots of happy thoughts!").
So for strange moods (especially possessions) to generate artifacts or new knowledge or whatnot that you wouldn't otherwise have been able to attain, at least not easily, would be very welcome. Doesn't have to be magical, just something alien and cool.