A little bit serious and a little bit silly:
THE SERIOUS: Have you ever needed to type just one Unicode character that isn't available on the standard iOS keyboard? Do you end up going to Google just to search for a character to paste into another app? Now you can have conveniently categorized Unicode characters right at your fingertips in a custom keyboard, and if what you need isn't there, just search for it using the search pane instead! Try searching by keyword or by hexadecimal code point.
THE SILLY: Make yourself stand out in your status updates and tweets! Flip your text upside down, cross it out, put it in small caps, or make it look olde-fashioned. Then attach a Unicode emoticon to it and text it to a friend, post it on your favorite social networking site, or copy it to your clipboard to use anywhere!