Banks provide a necessary service to customers, from those who simply have bank accounts to those who borrow money. Financial uncertainty, as the United States is facing today, can affect a bank's business, but strong banks will generally muddle through even the most difficult periods.If you are a dividend investor, the relatively high yields on offer from banks will likely be attractive to you. You'll want to choose wisely, however, with Citigroup (C 0.44%) providing an interesting example of why.Better th ...