There is a distinct advantage to filing taxes as a non-resident alien as opposed to a resident alien. Non-resident aliens only have to pay income tax on money earned from U.S. sources, while resident aliens (legal residents of the U.S. who are not citizens) owe tax on income from all sources, including foreign businesses and entities [source: Cussen].
One of the criteria that the IRS uses to decide if a taxpayer is a resident or non-resident alien is how many days they lived in the U.S. during the tax year and three prior years. So if you're a non-U.S. citizen and you want to pay less in taxes, there's a strong motivation to lower the official number of days you were in the country.
Here's where this particular exemption comes in. If you are a non-U.S. citizen temporarily in the country as a teacher or trainee (holding a J or Q visa), you can exempt all of the days you lived here. In other words, if you lived the entire year in the U.S. on a J visa while teaching at a university, you can still qualify as a non-resident alien for tax purposes. You would need to file Form 8843.
The same exemption covers an even smaller group of potential taxpayers: foreign professional athletes. If you are an Australian rugby player in the U.S. for an international tournament, none of those days count toward your total days of residency.