Saving a copy of your return as a PDF (in addition to the proprietary tax data format TurboTax uses) lets you open, view, and print your return using the free Adobe Reader or Foxit Reader.
This is a lifesaver if your tax data file gets corrupted or if you no longer have the TurboTax software to open it in. The government will also accept PDF copies.
Please note that TurboTax PDF files can't be "read" by the TurboTax software the way the tax data file can. This means they can't be used for amending, troubleshooting calculation issues, or for full prior-year transfer capabilities.