As time goes on it is only natural for a user to gain more personal data and files. Not all of this may need to be backed up into your FSS backup service. For instance, files that are already stored in another cloud service, such as iTunes, Google Drive, One Drive, or other services can be easily recovered and downloaded if there is a catastrophic failure. As an example iTunes keeps track of all the music you have downloaded and purchased from their store so you may not need to have a double backup of your iTunes music folder. Also, for some content that you are backing up, you may just not care about it anymore. For instance, that old college term paper from 15 years ago--you couldn't care less if you lost it.
How do I deselect a file from my backup set?