[ars]

It doesn't know which files exist on the server.

And the file can change on the server just as easily as the client. So how can it tell this without sending the complete list?

[ape4]

Yes in the general case. But in the case of a backup that's done daily the sender can say... there are all the files that changed since we last did this.

[ars]

Rsync is designed for the general case. It's useful for backups, but not designed for them.

[tnhh]

In this case, I think you want to be using unison rather than rsync.