Why is this online tool useful?
Believe it or not, spammers have lives too. They don't manually visit websites to gather new email addresses for the mailing lists, because ain't nobody got time for dat. Instead, they send out bots/webcrawlers (programs they have written) which follow links on websites and 'scrape' any email addresses they find for inclusion in their lists.
These web crawling programs don't surf the web using a browser like Internet Explorer or Firefox as we humans do. Instead they crawl the HTML code behind the page (which you don't see, unless you view the page source), and we can take advantage of this by obscuring the email address in the HTML, but making sure that humans who use website browsers will still be able to see and click on your email address.
The above tool converts your email address to HTML Dec code and then wraps in a script which you can cut and paste into your websites' HTML.