Assuming the premise that one needs to make mass promotions to fight retirements and minor league FAs depleting your rosters, the merits of promoting prospects late (during the playoffs, for example) rather than after rollover gives a way to "capture" the incremental bumps they've gotten outside of the typical "Regular Season" intervals.

The idea being that if a rating has gone from 64.3 to 64.8, the "Promotion" event would bump it to show as 65, and that would carry over to the following season, rather than just staying at 64.

You might call it cosmetic, or maybe it's real.  I think that's the crux of booger's question.