Your computer, phone or tablet retains your CaseManager data; synchronizing with Dropbox will give you have a cloud-based backup of that data. In addition, synchronizing with Dropbox permits to keep multiple devices running CaseManager.
CaseManager creates a link that communicates with Dropbox–you never have to visit Dropbox to synchronize or manage your data (indeed, it’s not a good idea to go to Dropbox to manage your CaseManager data because it can interfere with the handling of your data. A Dropbox account is free at Dropbox.com.