saving, synching and backing up

Hello,
I am confused between those.
I thought I understood, till I started Scrivener and it tells me that I have set my E-drive (external drive) to be synched, It tells me those files have been changed and do I want to synch them with my PC.
Huh…?
I thought by synching to the E drive, as Scrivener was saving on my PC, it would also save to the E drive. I work from one device only. How can those files be different? But the E-drive also has some old backups, and I don’t want them being synched back to my PC, in fact wiping new work.

Is there any advantage in synching if working from one device only?

Thanks,
Hetty

You might want to check the documentation. It provides very good information about these.

Shortly:
Syncing is used to export your project in a way to edit it on another device. After the external edits are complete, the files can be synced back into the master project. Scrivener provides two syncing modes, Sync with External Folder and Syncing with mobile Devices.

Back up provides full copy of the project. It is designed to restore the full project in case data, or project is lost/corrupted. It is a full copy of the current state of your project.

Hopefully it is clearer, now.