Why input captcha codes can earn?

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I know keep input those number can get pay. However, I want to know why those company need people to keep input for them? Is it because they need hackers? If not, who need the service of input captcha for them ? I can understand those input Brand name captcha's concept but cannot understand why need people keep input some number for them. Since, I cannot solve this problem, I dare not join those captcha site. Thank you very much!