I don't see the need for such complication on a captcha....
It's to make sure you are a human taking the survey and not a computer program. It's a deterrent to prevent anyone from using technology to skew the survey results.
The question is not why do we have captcha, but rather why such complication in the captcha... As I mentioned, there are other captcha questions that are not so complicated and subjective, like asking the person to type in the letters / numbers found on a photograph. I've sometimes been fed up with the visual captchas that I'll do the audio one instead. Those tend to be easier to complete and are not so subjective as to what you are to enter in.
That also answers the complication factor - for example, if Google can teach robots to
communicate to one another in an encrypted code no one can break, not even the designated decipher bot, and this program were to then be used and abused by a company in some way to skew results (not to mention the more terrifying implications this has for others who could get a hold of it and abuse it), then CAPTCHA creators have to compensate and make sure these bots have as little chance as solving it as possible while still attempting to maintain an ease of access for humans.
Granted that's an extreme scenario, and also highly unlikely to happen (at least not in the very near future), but situations in which bots become programmable to be able to "think" and "reason" like a human are why security measures online become increasingly difficult.