If you wrote a term paper 4 days ago that you accidentally deleted, you obviously would not choose a backup from a month ago since the term paper did not yet exist.
Conversely, if you are having problems with a file or accidentally altered a file that has been on your system for some time, but you aren't sure when it got corrupted, you can choose a backup from farther back to try to ensure you go back far enough to get the functional file you are looking for.