You will notice that whenever you have a wide angle lens on your camera, the image frame will include lots of background matter which is much further away than the main subject. Diffuser or not, the 1/r^2 law still applies, which means your main subject will look overflashed while the background will be dark.

The main reasons I use my diffuser is to create a mix of bounce and direct flash and to fully illuminate an umbrella used as diffuser.