The documentation is available here https://docs.mendix.com/refguide/attributes/
"Binary data. Can only be used for persistable entities as the data is stored in the database. For example an entire file. In most cases you want to use an association to a FileDocument or Image to store file contents."
The System.FileDocument entity stores the contents of its files in a binary attribute. I have not come across other reasons to use a binary attribute.
Binary