I quite commonly use it when I want to see an application behind the terminal e.g. I might be editing code in vim whilst the application or browser window is op

I quite commonly use it when I want to see an application behind the terminal e.g. I might be editing code in vim whilst the application or browser window is open in the background. Also, it just looks nice! By having an appropriate background image and playing with the opacity it has a negligible affect on readability.