Users login in to the Websites, Blogs, Forum and adding their content, comments, asking or answering the questions etc. If user has added his photo then it will get displayed besides his content but not everyone provides his photo image to the sites.
Most of the sites used to display a common avatar image for those who have not provided their image. When we read the content, it may be odd to see the same image representation for all users. Identicon comes for the rescue.