Personally I don't see it either. But as you saw from reading those drink package discussions, some people find it "convenient" to pay in advance. As said, doesn't make sense to me, but for them it does.
That said, I can think of other reasons as well. For example, because the price onboard is in US dollars, but most of those overseas pay in another currency, prepaying is a chance to lock in the current rates. That may be either as a safeguard against your own currency depreciating, or that you think it's a good time to buy and the USD will get stronger later (similar effects).