BFileSync does what other apps don't do, without ads. It allows me to make an on-device backup from one folder to another, including from the internal storage to the SD Card. I support people who have very limited computer skills, who work in an environment that is very challenging for hardware. If we configure their software to save to the SD Card, you can be sure that the card will fail, but if we configure for the internal storage, the device dies and takes the data with it. They don't usually have access to Internet, and when they do, it's metered. BFileSync allows us to support them with a local backup that can help us to recover their work when things don't go right.
The only thing I wish was easier was configuration. The ability to export and import a configuration would be invaluable for setting tablets up quickly.