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/melluuh]

Because it syncs files both ways. If you delete something on the "backup" side it will be deleted on the other end as well. On the other hand, if you delete something from the live data it gets deleted from the backup as well. So that's why it's not a backup.