Societies are imperfect and have a huge inertia, they need grey areas for compensating for those imperfections.

Cash is a way into those grey areas.

So that you can not have a phone, or be a hobo, be off the grid, go to a gay bar without leaving a trace, hold a controversial artistic/polical/religious event, let a beaten spouse organize an escape, mess around as a kid without your parents always knowing everything, try a product the state doesn't approve of and so on.