My view was over time the chassis does flex as pointed out by Bosswrench. The Sway bay keeps its shape but the mounting brackets wear and distort to suit the new rest state of the car.
You pull the whole mess apart- there is no apparent spring in the sway bar . You install new bushes and hardware that have tight original tolerances and it becomes very apparent how much the chassis has moved.
We are not talking allot here - perhaps a few mm each side , but it is enough to make fitting the bar back into the new no wear clearance supports a challenge when the A arm fixing points are all captive threads. Using clamps to pull the sway bar back into the rear mounts is very controlled and avoids overstressing all the captive bolts and nuts.
The clamps are very limited in the force they can apply - nothing like the huge forces generated during exuberant cornering. Any residual force in the sway bar after installation would be insignificant compared to this.
IMHO.