Branchwraiths are the oldest of the Dryads. They have ever served as the handmaidens to the Ancients of Athel Loren, and attend to the Treemen with a dedication bordering upon the fanatical. Yet before the coming of the Elves, the Branchwraiths were so much more. In those days, it was they who ruled the forest and they who tamed the Winds of Magic to bring sustenance to bough and branch. Thusly, few Branchwraiths think kindly of the Elves, and many hate them - it matters not that Durthu brought the Elves into the forest precisely because he knew that the Branchwraiths' cruel nature would lead Athel Loren to great evil. Now the Branchwraiths watch and wait for the seasons to shift once again. One day soon, they believe, the usurpers will be cast down from their lofty perches and the natural order restored.
The Branchwraith is a strong anchor to a Wood Elves frontline. Not because they are an incredibly dangerous combatant, but the utility they offer greatly increases combat effectiveness. One of their abilities is a bonus to melee attack in a radius which helps Eternal Guard and Dryads do more damage to their foes, bolstering the usually defensive frontline.
In Multiplayer, Branchwraiths will often be brought as the only spellcaster because their mix of life and shadow magic brings a lot of utility to the battle. Earth Blood to heal key units like Waywatchers or Wild Riders, Shield of Thorns to protect them, Pendulum to give destroy enemy infantry, or Melkoth's to slow down and damage fast moving targets.