1) Should you ever need to amend your prior year tax return, you will need that .taxyyyy data file in order to amend it using software. (ever since 2008, the data files always have the year attached to them "wahteverfilename.tax2016" for a 2016 file)
Certainly, when using the Online software, for 2017 you would be able to amend your 2017 file Online up-to ~15 October 2018, but after that you'd need either the special amend software, or the full-version download software (MAC users can only use the full-version downloads).
2) People who want to open their tax file, and look at the flow, also need the desktop software after the Online shuts down of that tax year.
3) we get thousands of folks who use the "Free" version and want to amend of look at their tax file the next year...they have to PAY for access to it.
4) people somehow lose track of the Online account, and then can't find their prior year file...neither the .taxyyyy file, nor the PDF file.
All that is avoided if you download both your .taxyyyy datafile from your online account AND two copies of your PDF file shortly after you have successfully e-filed it and had it accepted (one PDF just the filing copy, and another with ALL forms and worksheets)....AND Backed them up off computer. Don't trust that you will have access to them Online.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
