Let's say you have created a website design in Photoshop and the project consists of 3 files: homepage.psd, subpage.psd and blog.psd. And, you are using some common elements in all three like buttons, the background, etc.
Wouldn't it be great "only to update these common elements" and all the PSDs were updated?
CanLinkIt, a Photoshop plugin does that. It can link any PSD or image file Photoshop files to another.
The linked files are not updated automatically but this feature is triggered manually (with an "Update All" button) considering you may not want this synchronization to happen for some elements.
Also, for anyone designing a project as a team and using a file sync/storage service like Dropbox, you can edit one file while your teammate is working on a piece of that file and it's only one click to merge them.