Captchas were meant to keep spammers at bay. Unfortunately, that's no longer the case. Thanks to "cloud technology" like DeathByCaptcha - that is, people in countries where labor is cheap solving captchas all day - spammers have no problem getting through reCaptcha-protected sites and forums to do their mischief.

As a result, reCaptcha & co tend to be more of an annoyance to honest visitors than to spammers.