1 - Taxes. Best way to keep your money legally is to keep it at a financial institution that honors your country's tax system and keeps you from paying fines or serving jail time.
2 - Accountability. You'll have an easier time proving the money is yours from the legal forms they are required to file.
3 - Survivability. If you want your financial assets to transfer to your legal heirs you need legal paperwork!
4 - Accessibility. Financial institutions usually provide easier access to your money and have procedures for liquidating non/cash assets.