When working on your process or object in Studio, all tools you need are on the screen - tools on the left, buttons and menus on top. This is a great arrangement as long as you're developing or maintaining your processes.
However, when you are testing or doing a demo, all that stuff gets in the way and limits your real-estate on the screen. You need to have access to as much space as you can. Typically, you don't want to maximize BP as other apps the process interfaces with are also on the screen.
Wouldn't it be nice if you could click away the toolbar on the left and the menus on top? Customizing in style with Office, Eclipse, RDi, Visual Studio, ... - all other IDE's!
Now, you might argue that if there's too much on the screen it is about time to split some of it off into new pages. Correct, I agree to that. Unfortunately, we still have to live with the legacy stuff that was made in the early BP years and we're not supposed to re-factor everything we see when we're only going to do a smal tweak to the process.
Also, for a demo it would be nice to have a clean screen only displaying the process itself, without all the extra's in order not to divert the attention from what you're trying to demo.