The aim of this check is to protect the owner of the e-mail address from receiving unsolicited e-mail.
Although spammers can rent or buy existing e-mail lists, many opt to use software known as 'e-mail harvesters' (often referred to as 'spam bots') that extract e-mail addresses from web pages. These e-mail harvesters work very much the same way search engine spiders do and try to collect every e-mail adress they encounter on the web. However, the brute force algorithms they use, are not able to answer the simple question above. So, basically, we're testing whether you're human or not.