An avatar is the image that you place next to your name to accompany it in your postings. It is a very useful thing to have so people can recognize your posts at a glance because it is much bigger than your name and the mind does not have to go through the process of translating the letters to make sense of them and then tie them back to you. It also breaks through the sea of orange on instructables (the colors may differ on other sites) so people can see that you posted it at a glance. If you have the general pic of a person image or a commonly found picture on the web, do you think people will notice your posts as much.
An avatar after being used for a long time, becomes a surrogate face for you on the site it is used on.