Small World : purpose of hiding VP
In Vinci, the original game that Small World is based on, VPs are open.  In that game it can lead to end game kingmaker effects where a player can't possibly win, but can decide who does win based on who they attack.  With hidden VPs, the kingmaker effect is still there but its less obvious, as players who aren't precisely tracking every VP won't know when a particular attack will make a difference of who wins.