Just look at this typical Sub-Launcher's usage scenario: While using a web browser you need to use calculator. Press the Sub-Launcher's trigger to switch to your favourite launcher. Launch calculator as usually, and do your calculations. Return to web browser whose state remains totally unchanged! Without Sub-Launcher you had to quit web browser, switch to calculator, and then launch web browser again. This caused web pages to load again, or worse: same data submitted twice to the server. With Sub-Launcher you can use your favourite launcher to sub-launch any application - the web browser & calculator combination is just an example here.