Dropbox to allow a setting in the Preferences > Sync where a user can nominate a modified file date in the past (i.e 30, 60 or 90 days) where if the file modified date exceeds this. the files get automatically removed from the persons device allowing people to manage hard drive space more efficiently. Where the no files are left in the folder, the folder itself then gets removed from the device. To put it back on, the person would need to resync it and save it as a updated modified file.