- Precise focusing for macro, landscape, product photography;
- Can really help with focusing when using a teleconverter and the camera is no longer able to AF the usual way - great for wildlife;
- Can frame your composition at strange angles, when it would be impossible to look through the viewfinder;
- Can tether the camera to a PC/laptop and view and shoot remotely - even wirelessly if you have the right accessory;
- Have the option of a Live Histogram to help with exposure setting;
- Have grid lines to help with levelling horizons;
- Can AF anywhere within the scene (50D and later);
- I've never tried it but you can even use Face Detect AF (50D and later);
- You can shoot very quietly.