Sometimes a browser needs to leave a little data on your computer, just a little 5-10KB nibblit, a cookie.
HTML5 is a hungrier beast than that, infact, the HTML5 Web Storage standard was developed to allow sites to store larger amounts of data (like 5-10 MB) than was previously allowed by cookies (like 4KB). localStorage is awesome because it’s supported in all modern browsers (Chrome, Firefox 3.5+, Safari 4+, IE 8+, etc.). But that's where it should end. But that's where it should end.
Thanks to a little HTML5 vulnerability, however, this site can and will fill your entire hard drive with trash. Specifically cats. Lots and lots of cats.
I can [not] think of so many legitimate uses for it.