Sign In to Participate
From Social Patterns
Most communication and activity tools in social contexts require sign up before participating (see Sign Up or Registration).
This serves multiple purposes. Users trade something of value for the privilege of participating. Registration information is used for monetization and advertising by site creators.
Additionally, requiring sign in, allows the site owners to be able to save the contributions of each user. Without the registration information associated to the user, the user’s participation is lost once the session is over.
Finally, having an account associated with a particular username helps builds up a reputation based on participation and allows others to form an opinion about that user and their contributions.