NOTE: Rainwater Harvesting is illegal in a few states. Where I live, California, it is not illegal. Please do a search on your state and the laws.
Mostly, rainwater harvesting is seen as a way in drought stricken states to have water to use for your garden and plants in times of rationed water. Southern California has experienced this very thing. The former governor tried to make “Brown is the New Green” into a catch phrase. As in, having a brown yard is better than that green yard that sucks up water we need to use personally. It is why I put in a drought resistant yard – see my About page.
While it is not typically for personal drinking water, if we were to have a major earthquake where water was completely cut off from the area, or massive water rationing from severe drought, no one would fault you for using the water for personal purposes. You would just need to know the ways to clean your water to make it drinkable. It is, after all, most likely to come from your roof. That means roof detritus, bird poo, critter droppings, dead bugs and all other kinds of stuff.