Workspaces save the content and positions of all open windows so that you can restore them later. This is ideal if you are working with the same set of windows over and over again. Save these windows as a workspace and recall them whenever you need them again.
DEVONthink is one of those apps where you really could discover a new feature almost every day, if you wanted to. I’ve been using DEVONthink for a few years now, and was completely unaware of the workspaces feature. Although I mostly use the widescreen four pane view (with my PDF or file displayed in the far right pane), workspaces could be handy for different workflows. One that comes to mind is when you need several items on the screen at once.