The advantages of running tubeless are:
Less weight: A tube might not seem very heavy, but rotating weight that far out on the wheel is noticeable. So no tube along with Alienation's lightweight casing has a significant effect on the feel of the bike.
Less flat tires: With sealant constantly flowing inside the tire, small punctures are quickly sealed. There is also no worry of pinch flats.
Lower tire pressure: Although the 138 is rated at 85 psi max pressure, it's possible to run much lower pressures without any worry of pinch flats. Lower pressure can give you better traction, control and smoother ride by allowing the tire to conform more to the terrain or obstacle.
To run the 138 tire tubeless, it is strongly recommended that you only use it with a TCS compatible rim. Alienation's proprietary TCS (Tubeless Compatible System) bead ensures a perfect fit that can be inflated and seated with a simple hand pump. You'll also need TCS compatible rim tape, valve stem and sealant.