If your website allows people to add content, leave comments or fill out forms of any kind then I highly recommend that you use some kind of spam catcher to prevent spammers from introducing dangerous content to your site visitors.
It’s a sad fact that most spam is generated by bots, not humans. Therefore, if you allow a doorway for their malicious content then you are likely to be an easy target and you’ll receive more spam comments than you can ever filter through and kill on your own. That’s why you need CAPTCHA.
CAPTCHA stands for “completely automated public Turing test to tell computers and humans apart.” There are a variety of these tests online, but they all have one thing in common: They are easier for humans to fill in correctly than bots.
Because CAPTCHAs are difficult for bots to fill in, they filter out the bot generated spam comments so that you don’t have to. That doesn’t mean you won’t get any spam. You’ll still get human generated spam, but there is a lot less of that than the bot generated spam. Get the CAPTCHA. You’ll thank me later.