The simple fact is that you cannot be too careful when it comes to online security and protecting your customers' accounts. There was a time when using a simple username and password combination was quite adequate as a means to secure an online account. Today, these are easily hackable as most people use easy to remember information for their username / password combination. When you add a second identifier to the mix you are creating a two factor authentication system that is virtually possible to hack.
Banks Use Two Factor Authentication Security
Banks and the banking industry have long been the target of hackers who will hack into a person's bank account, take all of their money and their banking information so that the can use it to steal as much as possible before they are discovered. In light of this most banks now use the two form authentication system to protect customers' accounts.