It a compromise to having the complete seatmast you’ll see on road bikes. I believe it allows the frame designer more control over the desired feel of the frame by eliminating an external factor like a seatpost. Yes its lighter however the supplied carbon seatpost is a normal 330mm length.
Official trek spiel:
Seatmast systems are lighter and offer an added level of compliance to a frame, allowing the rider more control and less fatigue. However, traditional seatmast systems require cutting the frame to achieve the perfect fit.
With our no-cut system, riders can get the performance advantage of a seatmast system with the versatility and adjustability of a traditional seatpost.