If you shoot infrared, the "infinity" point for IR is actually farther than the infinity point for visible light. A lens that did a hard-stop at the infinity point for visible light wouldn't be useful for IR imaging.
Similar... red, green, and blue light focus at different distances and if you use filters (especially narrow-band filters) you would want to be able to compensate for this.
There's a little margin for error in each copy of a lens or a camera where the sensor might be shimmed fractionally closer or fractionally farther.
There are even thermal expansion properties that can shift focus ... metals contract when they get cold.
So there's no single reason, but rather a lot of reasons why it's a good idea to give the lens a little wiggle room. And think about this from the financial standpoint of the lens maker... if they did a hard-stop and for whatever reason that "copy" of the lens happened to be just shy of focus on your particular camera body copy then that lens has to be returned as "defective". But with the wiggle room... that saves them a warranty claim.