While you may be able to handle many of your financial transactions over the Internet even when deployed overseas, you may encounter a few situations in which you'll need someone to attend to something in person. You'll need to give someone the authority to act upon your behalf while you're away. Most financial institutions require proof that you've given this authority to someone else before they'll complete a transaction.
If you're married, you can create a joint account at your financial institution. That will allow your spouse to handle financial issues in your absence. For unmarried service members -- or those who wish to retain their own personal bank accounts -- it's a good idea to designate a trusted friend or family member with the power of attorney.
Power of attorney is a legal designation that allows you to appoint someone else to act in your place in financial or legal affairs. You'll need to be able to provide the certified documentation proving you've appointed someone else with the power of attorney.