- Saving money in any currency means you get no interest/dividends on the savings, which if inflation is greater than zero means you are a actually losing value.
- Add to that casualty risk (fire, theft).
- Add to that currency conversion costs, which can run to several percent (which is not a small number).
- Add to that exchange rate risk, though that may also work in your favor; it's hard to predict which direction that will swing.
Basically, all the same reasons you might not want to keep piles of your own country's cash, plus or minus the exchange rate question.
Banks exist for good reasons. You probably want to use them unless you are explicitly playing the exchange rate game -- And if that's what you want, there are probably better ways to do it.
If you need help not touching the money, CDs or other term accounts might give you enough disincentive. Or might not.