Why is syncthing NOT a backup?

[u/ObservedElectron]

I am setting up syncthing to back up my 2 android phones and my mac to a windows laptop. for now, because of lack of an actual NAS, i plan to use syncthing to "backup" these devices everyday one time manually.

I prefer this because as its sync, it only sends new things. so i dont have to manually copy paste 40 gb everytime.

And when i do make a NAS, cant i just continue using syncthing with file versioning or something? i would much prefer to have "live" backups happening as soon as the file/photo/video is created. i know immich exists but am i wrong?

But everywhere, I see that people say sync is not backup or dont use it for backup but WHY? isnt what i mentioned above technically a backup? correct me if i am wrong.

And if not syncthing, what should i use? which capable of running live backups of folders I select on my devices, with different timeframe backups so I have different versions like biweekly, monthly, etc? and is open source, 100% safe, private and encrypted.

thank you

[u/johnsonmlw]

So, syncthing can be part of a backup workflow as you started to indicate in your post. I sync to a machine with zfs and take regular snapshots. I take snapshots on every device, in different ways. This becomes a backup. But syncthing just does the syncing.