1. They are a final exam of sorts. They should challenge you on the things you have learned up to now.
That's quite right, yes.
Ironically, it's in MMOs like World of Warcraft or Wildstar that I found bosses matching that description, to some extent.
I say to some extent, because due to the nature of MMOs, you can find strategies to beat bosses on the internet and sometime completely miss that the game tried to prepare you all along. And sometimes, instead of thinking "hey the game prepared me to fight this boss, I saw those mechanics before!", you'd instead think "hey this quest is a training for boss X".
Anyway, thanks to quest lines, dungeons theme, or overall knowledge of the lore or ingame mechanics, you can enter a boss room and immediately get that he will do this, probably will do that, may enter a second phase where X happens, etc.
And when you're right, it's kind of very satisfying.
