Because if you want to have a watermark on images that you are going to share with a Client you should export those image as JPG file and at that time Add your Watermark. Then place them in a collection and Sync that collection with the Lr Web system and then Share that Synced collection/gallery with your client.
Yeah, that’s what I have to do at the moment. I just thought that if you can add a watermark on export, maybe there was something I was missing whereas you can add a watermark when sharing. Save all the extra steps of exporting and importing again.