Just by being involved, engaged, inclusive, a portrait of the self and a person's identity is built within the system which can then be interacted with by others.
The user needs to get oriented and learn how to use or interact with a new site, application, or experience for the first time (or after a long time away).
The user needs to choose how to identify herself online, in association with a contribution or on a profile page, module, or badge.
Users need to know who else is available or present in a social context, or to broadcast and share their own presence status.
A person participating in a social structure expects to develop a reputation and hopes for insight into the reputations of others.