The keyboard shortcut associated with the Focus Editor item in the View menu is a quick way to set the keyboard focus to the main editor. Doing so will make the text cursor (blinking vertical bar) visible in the main editor allowing you to continue editing the file.
Using the Focus Editor keyboard shortcut allows you to quickly switch back and forth between the main editor and other panes. E.g. while working with the search and replace pane, you can press Tab on the keyboard to cycle through the various controls on the search and replace pane. To switch to the main editor, press F9. To switch back to the search and replace pane, press Ctrl+F.