It is the pressure barrier for the flowing stream, and it can be pulled without drilling a new well. Diameter is a velocity choice. In a gas well, too large a diameter drops the velocity until liquids fall back and load the well. Too small a diameter, and friction consumes the drawdown that should have been at the sandface.
Tubing is not casing. Casing is cemented and structural. A packer usually seals the tubing to the casing so the annulus is isolated. Tubing-head pressure is the pressure at the top of this pipe.