Well, it's not strictly necessary, I could have just used an image with my email address but that could be harvested by OCR-capable spambots (I think there are a few in the wild) and wouldn't be clickable. ;)

With the captcha, I can be fairly sure that a human solved it (reCAPTCHA is particularly good at filtering bots, afaik) and the user gets a clickable link.

In any case, that's just to accomodate browsers without Javascript or with it disabled. I believe most people have Javascript enabled, so they'd get a regular mailto link.