Why avatar is showing in all forums?

If you question is why we need avatar, it is because a picture speaks thousand words. An image helps good identification of the person or the topic, instead of reading the heading or profile name every time. For example, when you see the header of mylot, you instantly know which topics has the latest discussions, instead of reading the name of each topic. Same concept can be applied to user profiles. You can instantly know which of your friends have replied on the discussion instead of reading through all the names (which also takes time). If your question is why use avatar to show virtual image, then you effectively you have answered your own question. the virtual image is called avatar.

Avatar represent you. It's a virtual image of yourself after your username. You can put any photo that you wish as long as it fit the qualification stated by the forum you join. Sometimes it's not enough by your username alone to show who you really are.

In my opinion, avatar is your personality. Like in thi mylot, I put "make money be happy" as my avatar. That means I make money in mylot and than be happy with discussion.

I'm not sure I really understand your question since to me an avatar IS a virtual image or photo of you...isn't it?

Avatar makes it easy for one person to be identified before the username is read. So in most cases, avatar must always be unique. If you see users with same avatars, what will come into your mind? You will think they are one of the same person.