The Soid ecosystem aims for decentralized web, in which users are in control of their data, and identity. To enable that, it uses WebIds as the primary identifiers for agents. The WebId Speciication describes it's motive as follows:
A global distributed Social Web requires that each person be able to control their identity, that this identity be linkable across sites - placing each person in a Web of relationships - and that it be possible to authenticate globally with such identities.
A WebID is a URI with an HTTP or HTTPS scheme which denotes an Agent (Person, Organization, Group, Device, etc.). For WebIDs with fragment identifiers (e.g. #me), the URI without the fragment denotes the Profile Document. For WebIDs without fragment identifiers an HTTP request on the WebID MUST return a 303 with a Location header URI