You may have to mail in a 1040 form even if you don't owe the government any money. The issue for the IRS isn't how much you owe, but how much you earn: Everyone above a certain level is supposed to submit a return. In some cases, you'll put more money in your pocket by filing.
You have to send in a return if your gross taxable income from all sources is more than the IRS cut-off level. For example, a single filer under 65 has to submit a tax return for 2012 if his income is $9,500 or more. If you're self-employed, the gross income from your business equals total sales minus cost of goods sold. If you provide services rather than products, your gross income equals your total income, with no deductions for expenses.
Even if you claim someone as a dependent, she may have to file her own return. As of 2012, if she earns $5,800 or more, or unearned income of $950, she needs to file. Unearned income includes interest, dividends and capital gains. If your dependent is too ill or too young to submit her own return, you have to do it for her. If the dependent is your child, and you choose to report her income on your own 1040, she doesn't have to file.
Sometimes you have to file even if your taxable income is well below the cut-off point. If you owe alternative minimum tax, the IRS wants a return. If you work for tips and didn't report them all, you need to submit a report to pay Social Security and Medicare tax on the unreported income. When you work for yourself and make a net income of more than $400, you pay self-employment tax, even if you don't owe any income tax.
Sometimes filing when you don't have to works to your advantage. If your income for the year is less than the cut-off but your employer took out withholding from your paychecks, you have to file if you want your refund. There are also tax breaks such as the earned income tax credit and additional child tax credit that pay you money if the credit is greater than the tax you owe. You can't claim the credit unless you file.