LR Classic and syncing with CC

Hello. For many years I've had Lightroom Classic with approx 50K images. Over the last few years I moved to CC and am up to approx 20K images there. I've also moved to a new Macbook recently. I decided to create a new catalog (the old catalog was running very slowly with all sorts of mess) and imported the 'classic 50K images'.  I store all these on an external drive.

Under collections in classic, I can see 'From Lightroom' and all the various albums from CC are listed there. 

Now - under Classic Settings, Lightroom Sync, I ticked 'Specify location for LR Synced images' as a 'Sync' folder on my external drive.

The question is - what does this option do - because now I'm seeing - under the Sync folder, a massive list of files being downloaded - that appear to be a combination of CC images AND photos taken with my mobile which I never did put into CC. I'm a bit confused about this.

If I unselect this 'Specify location' - what does that do? 

Any help with this much appreciated - thanks.