https://www.random.org/ generates random numbers, that you can use for testing.
Why it is important? Humans tend to perceive randomness differently than machines. For example people were asked to give a number between 1 and 20, the most random number was 17. Read more at is-17-the-most-random-number. See also http://www.dailymail.co.uk/home/moslive/article-1334712/Humans-concept-randomness-hard-understand.html.