With Wi-Fi you can sit in your house and take pictures with your camera mounted somewhere else.
Actually, in theory, you can control your camera from anywhere in the world (of course, there are issues with this, such as sleep mode, battery power, etc.). But the point is that you can climb a tree, mount your camera next to an eagle's nest, then go home and start taking photos when the birds come back.
Also, it allows your camera, in everyday situations, to communicate with your smart phone. So even if you aren't in a wi-fi zone, your phone can be set up to instantly store the photos from the camera. Then you can send the photos out to anyone or any website you choose.
So, for example, you can technically take a photo of your friend and say, "Check your phone. It'll be there in a couple minutes."
Yes, this can be done with a cell phone camera, but the quality isn't the same.
NOTE: here's what I'm really excited about: I just ordered the EOS-M a few minutes ago. I'll install a wi-fi card (eye-fi) and then I'll be able to actually use the camera's touch screen feature through my Iphone. Every single setting will be accessible.