The Spirit tree focuses on creating witch doctors whose main role in battle is to have better mana regeneration and therefore more spells can be castbetter spellcasters, to be good at controlling crowds and to be more adapt at surviving than witch doctors of other builds. Also, like any skill tree focusing on any element of gameplay, it cannot be called a skill tree if it doesn't have signature skills that can actually kill. Luckily for us, the Spirit tree has a few cool skills that makes enemies easy to kill, like Haunt, Soul Harvest and Mass Confusion.