Word Count: The website does exactly what it says it's going to do. It gives an accurate count of the number of words in the articles you write. This is especially helpful when you are writing blog posts for others or need to write an article within a specific range of words.
Character Count: Much like the word count function, the tool also gives a character count of everything you write. Again, this can be quite helpful for certain writing assignments and guest posts.
Keyword Density: This is an especially useful extra function. In addition to word count and character count, the tool also list the word density for the top 10 keywords in the article you have been writing. This allows you to determine if you have been using words too often in your writing and to make sure that the articles is hitting the keywords that the article is about.
An added bonus is that you don't have to go to the site to use this tool. There are downloads for Wordpress and Google Chrome so that you can use the functions with your regular writing within these formats. If you write articles on a regular basis, I think you will find this to be an excellent tool that you will use on a regular basis.