Captcha tests if you are human being or not and tries that by giving you words or phrases in a form quite confusing to test if human can recognize the letters and characters to determine what exactly is the word.
Although nothing can prevent this from determined hackers from cracking the capthca the purposes of the captcha were
- To determine if you are human or not by testing you with phrases or words and numbers with confusing characters. OR
- To type words into books online simultaneously with millions of others without realizing.
The people who designed this found it easier to crowd-source this book typing project, help digitize books, kill two birds with one stone.