What characterises classical digital identities, and what are their strengths and weaknesses?

The identity service knows the user, authenticates them and is involved in every access to a relying party; this gives good usability and usually adequate security, but it enables profiling that is hard to prevent technically and makes the central service an attractive attack target.

Strengths:

• Usability: one login flow the user knows; password reset and support are handled in one place.
• Security: usually sufficient, because a specialised service can afford proper protection and multi-factor authentication.

Weaknesses:

• Profiling: because the service takes part in each login, it can record when and how often a user visits which relying party. Preventing that at the technical level takes very large effort.
• Attack target: a central identity service stores the user data and credentials for many relying parties, so a single breach has a large blast radius.

These two weaknesses are the motivation for user-centric models: take the identity service out of the usage path.