I would think hardening and premature tip aging would be the biggest "reason" some might have for thinking it is a necessity. I only ever do it (rarely at best) if I am trying to keep some chalk out of the case or off the shaft, but when you think about it, chalk is a desiccant, meaning that it absorbs moisture, and would I suppose have the ability to draw whatever moisture was in your leather tip out, and thus create a drying and hardening of the tip, this is in fact the reason old tips get changed, although, I don't think that wiping or not would have much if any measurable effect on the lifespan of an often used cue....
D