FSA's are a great way to pay for certain personal expenses with pre-tax dollars. The two most common items to pay through an FSA is unreimbursed medical expenses and dependent care expenses.
Here's why FSA's are so great. Let's say you're in the 25% federal tax bracket, live in a state with a 5% tax, and are still paying the full FICA and Medicare taxes. By taking advantage of the FSA, you can allocate $100 of your salary to pay $100 of eligible expenses through the FSA. The only other option is to earn $166, pay $66 in taxes, and end up with $100 to pay your bills.
So with an FSA, $100 = $166. It's a total value play.