Not everyone wants to retain Windows 10. If you keep the Windows.old folder you'll be able to revert to Windows 8 at any time within a month of installing Windows 10. So it might be best to keep it.
If you don't go back to Windows 8 within a month, the Windows.old folder will then be automatically deleted.
If you
really want to get rid of it straight away, make sure that you use the correct method:
http://www.cnet.com/uk/how-to/how-to-delete-the-windows-old-folder-from-windows-10/