I am somewhat of a newbie to WordPress as far as extended functionality. I use WordPress as a base for almost all my sited and recently designed on that could really benefit from users being able to create their own account. I want the users to have an account so they can post comments and have their name or username appear when they leave a comment.
most sites will enable comments and have the Gravatar addon. Most people use gravatar now, so this lets people leave a comment with a name and website address – and have their custom avatar without signing in.
For posting content – that’s when I think you need to be looking at user accounts. Most sites would give someone a limited "contributor" account, so they can edit their own posts and post new ones, they don’t see anything else of the WP admin panel.