Let's say you're syncing your personal files into another location once a day.
On Monday you delete files. On Tuesday you edit a file. On Wednesday you maybe get some malware that (unknown to you) encrypts some files (or all of them).
A week later you realize that things went wrong and you want the deleted files back, or the old versions of the file you edited, and of course you'd want back the files that the ransomware has encrypted.
If you simply sync files you have no way to get back deleted files. Every day it synced whatever was in there, overwriting what was there before. If you also sync deletions then sync deletes the files. If you don't sync deletions then files keep piling up when you delete them or you move them around.
An incremental backup system like borg looks at small file chunks, not at files. Whenever a file changes, it makes a copy of only the chunks in it that changed. That way it can give you the latest version of the file but also all the versions before, and it doesn't store the same file over and over, only the chunks that really changed, and only one of each chunk. If you move a file to another folder it still has the same chunks so borg stores that it moved but it doesn't store the chunks twice. Also if several files have identical chunks, those chunks are only stored once each. And of course it never deletes files unless you explicitly tell it to.
Borg can give you perfect recall of all past versions of every file, and can do it in a way that saves tremendous amounts of space (between avoiding the duplication of chunks and compression).