What are these files?
I assume these are backups made from sync.?
They are usually labeled:
ProjectName-bak.zip,
ProjectName-bak1.zip,
ProjectName-bak2.zip and so on…
Does it mean my sync failes somewhere and backup is being created?
Or are these the extra copies made for conflicting files?
Can someone shed some light on these?
It looks as though you have instructed Scrivener to back up your projects to your Dropbox folder. You can check the designated location by opening Scrivener’s preferences. selecting the Backup pane and looking at the file path listed at Backup location.
If your backups are stored in the same folder as your live projects, you should change the backup location. I would never store backups in the cloud myself (as in have that as my only backup location), but that’s a personal preference.
Don’t you mean you never store your live projects in the cloud? 
That aside, I have my live projects stored variously in Dropbox or Sync, with zipped and dated back ups for frequently used projects stored in Box Drive, and less regularly used back ups stored in iCloud.
I also clone my internal drive to an external drive virtually every week.
But you should never have your live projects and your back ups in the same location.
Mark
Nope, I meant I would never use the cloud as my only backup location:
Excellent advice, much more succinct than mine
Neither.
They are backups, copies of older versions of your project, made for safe-keeping, usually every time you close the project (depending on your setting in Preferences). So if something goes wrong, you can go back to a previous version of the project.
You should check your setting in Preferences -> Backup and also read about it in the manual so you understand what you are doing.
4 years of Scrivening and I only find about this now. I feel ashamed of myself 
But thank you all for a piece of amazing advice!
My backup was the simple Save to Dropbox & then save to local HD manually, never even thought about an extra option for separate Backup, or even considered the differences between the too lol
Knowing now that there is a separate function specifically for Backups gona save some clicks
Two of the beautiful things about Scrivener are its automatic saving of the working project and its automatic backup routine.
Open, write some, wait a few seconds, close the project, and all you wrote is safely both saved and backed up.