What do online employers get with advertised captcha tasks?

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Pardon my innocence and ignorance for I am a newbie in online part-time jobs. Upon venturing into this type, I had experiences in doing typing into a field images that pop up onscreen once I download a specified app. Lately, I learned that this is called "captcha" task. I wonder why employers will pay for a specified number of typed images. What do they get from offering and paying for such tasks?