I have two monitors; one I use for playing full screen games, the other I use for pretty much everything else.
I'd like to be able to browse the web and play a full screen Spiral Knights at the same time, so I can use resources like the Wiki. However, the Fullscreen mode locks my mouse inside one monitor, and won't let it go to the other.
I could play the game windowed, but that's no fun.
A windowed fullscreen mode (essentially fullscreen without the above limitation) would be nice.
There are ways to do this currently: http://forums.spiralknights.com/en/node/11550 But would be nice to have it as an actual option.
In full screen it takes a couple minutes to switch to your browser to search all the inappropriate images you want when you should be playing.
In windowed mode it is not fun for you.
In full screen mode your mouse cannot misclick to accidentally close the game or to another program when you want to be fighting something.
In windowed mode it is too difficult for you.
In your options you can change the window size to various preset sizes.
If you are not doing anything which requires immediate attention in the game then you can take the time to either switch directly from the game while it is in full screen or go to your options and set it to be windowed until you no longer need the browser.
In space no one can hear you scream.