Or simply you misplace your wallet.

Then you have to make the decision of how much you're willing to risk in taking out of your "totally safe" under-the-mattress stash each day, and that runs into inconvenience of if too high you lost a lot after a few beers on a Friday night, if too small you can't buy everything you wanted in the spur of the moment.

Or just the power of the state behind them. The government can always take your money, regardless of what form it is in and where it is.

Or a legislature.