The Samsung Device app is developed by Samsung. It gives your device access to the contents of your Dropbox folder. It syncs the contents of your Dropbox folder to your device's built-in photo gallery. It has a 'DocumentSync' feature on your phone which allows you to add, edit and delete any files appear in this folder. So, Samsung Device app also sync the contents of your Dropbox folder to a DocumentSync folder.
Once you enabled the Samsung Device app, any synced Dropbox files you delete or modify from within your device's gallery or DocumentSync folder will be changed or deleted in Dropbox as well. So, it is better to disable the Samsung Device app. This app is separate from Dropbox and can be disabled without affecting Dropbox. You can also disable this app from the Dropbox website. Dropbox allows you from the website to see who added, deleted or changed a file by looking at the previous version history.