Parallax is the observed displacement of an object caused by the change of the observer's point of view. In astronomy, it is an irreplaceable tool for calculating distances of far away stars.
IMO adjusting eyepiece (ocular) focus on the reticle while looking at the sky, catching your eyes napping, not allowing your eyes to auto-focus, etc. is error-prone. By removing parallax first (and focusing objective on reticle plane) you are then free to adjust ocular for sharpest target image without any gymnastics to catch your eyes napping. When the target is in sharp focus, so must be the reticle as well if parallax error has been been removed.