Acts are useful in both cases, moving location to a new map doesnt need explanation, but changing the act and keeping location can as calel mentioned free up you entity budget, but also enables you to create new triggers - conversations, quests, etc with the npcs, maybe move or add mobs, that sort of thing.
you can with some patience create a nice branching storyline by moving to different acts depending on player choices, just dont expect to do it overnight
still want some global variables to use though -)