If you know how to set up your own wi-fi, and you're not a complete moron, it's pretty easy to open up the network paths, and you can open up anything in another computer that's connected. (if you set up your wi-fi as home, and allow the public sharing option)
Mostly, this is used to pass on files, music, etc. if you just bought a new computer, just let it set for a few and all your info will be there soon.
But, you can also use this to steal files and such from others, or place files into their computer.
So say... I have this nice little program, or virus, I can put it on your computer if you try stealing my houses wi-fi.
"If you must mount the gallows, give a jest to the crowd, a coin to the hangman, and make the drop with a smile on your lips"