This tool capitalizes sentences - makes the first letter of a sentence a capital letter. If you have some paragraphs where the sentences are not capitalized then you can quickly use this tool to get the proper capitalization.
It also can correct text where the caps lock key has made every letter capitalized. You've accidentally pressed the key and when you look up all your words are capitalized. It can be fixed easily by using the online tool below.
You can also use it to change the first letter of every word to uppercase - for a headline style capitalization. Though probably not as handy as the other capital letter changing functions
And who doesn't hate when you accidentally caps lock a whole bunch of letters... fear not your capitalization woes are over ;)