Widget Manager manages your widgets. Not really. Actually YOU manage your widgets via your widgets sub-tab. What this lets you do is copy the widget config from a blog you pick as your "mother blog" to any and all other blogs. Useful if you want the same, or basically the same widget selection on all your blogs is the thing. Like, copy the "mother" config over to a new blog, then do minor tweaks to make the new blog be exactly what it needs to be.