system-wise, revelry is something your char must do if they get mixed result from loot roll and he/she can't take more gold
revelry may result in bad things, for example, my char's Revelry in Occult might attract a powerful spellcaster to raid the dungeon
additional info:
Loot Rolls resulting in Failure starts a Revelry, each Revelry fills one Dark Heart
Revelry being done when your character has more than two Dark Hearts results in Overdo, that's when the bad thing happens in Calamity phase just after Downtime