Well, not everyone is lucky enough to have an unlimited data plan, for us, Wi-Fi is a life and bandwidth-usage saver! I, for example, have to do with a 100 MB plan for now. Luckily, I'll be able to upgrade to a better plan once my contract is up - I bought my Hero 13 month into my two year contract, I had no data before, so it costs extra, this was the most reasonable and cost efficient option until I can switch to a data included plan - until then I rely on Wi-Fi for heavy data moving.
But it has other benefits, too. I have a decent cable internet connection at home, so downloading apps, podcasts, etc... is a breeze. I set up an FTP server on my PC, and a client app on my phone to copy files without having to bother with a USB cable and mounting the SD card. Heck, I can wake my PC with a single click from my phone, and I can use it as a frickin' remote control!!! How cool is that?!?

There is Wi-Fi network coverage on my campus, and if I connect to it, I can reach the internal network of my university without having to type in passwords. That's just what I use it for, I'm sure there are many other things you can do with it, besides the obvious "conserving-my-data-usage".