On Android, apps can access each other files by default. It is not secure, as you may have files that are not meant to be shared with anyone else. That's where the 'Private downloads' folder comes handy.
'Device Downloads' is a regular folder on your device, which is seen and used by other apps and file managers (including file transfer to PC).
'Private downloads' folder is stored in Aloha app and is completely private and secure. You can keep your sensitive files over there.