The reason would be that the cue stick is not the same in all rotations and you want to always use the same rotation for consistency.For what reason?
No cue is perfectly straight. No tip exactly the same in all rotations. Maybe the rotation of the spine makes a small difference in squirt (reports on this are mixed).
Whether the reduction in variables is worth the bother is a good question.
I marked my ferrules for a long time and always played with the mark up. Getting the rotation right was part of my pre-shot routine and pretty much subconscious. Maybe it helped get a flow/zone going. I'm lazy now and don't do it.
Many snooker players always shoot with their cues rotated to a specific point. Ash shafts kind of need this because of their heavy markings.