With S.O.Linker, Photoshop Smart Objects are linked to external Photoshop (.PSD) files. This provides enormous time savings to web designers when changing or editing a Smart Object contained in multiple page layouts.
For example, let's assume the same Header is used in multiple web pages - a common occurrence for most web sites. It makes more sense to convert it to a Smart Object and move it to an external .PSD file.
The problem is that Photoshop is unable to update the Smart Object automatically when the external file changes (unlike InDesign).
For example, if you slightly change the Header, you need to manually update each Smart Object (the Header layer on the screenshot) of each web-page layout. This is both time-consuming and prone to error.
With S.O.Linker, changing the menu font size, for example, is done just once and all the page layouts are updated with a single click of the mouse.