Say you are working on a build with a repeating pattern, but you want to change the pattern. Right now you would have to do this manually block by block or template the sections and replace the entire build. 
With find & replace you could select the chunk of blocks you want to replace, and this is stored in memory and is maybe shown on screen as a reminder of what the old pattern was, then you make the change you want and select the new pattern and confirm the replacement of the old pattern with the new one within the template. Many hours of busy work avoided. Rejoice. 