Rebuilding primary keys in a physical diagram updates primary keys for tables.
Rebuilding primary keys is useful following the reverse engineering of a database in which all of the primary keys could not be reverse engineered, or if you did not select the rebuild option for primary keys when you reverse engineered the database. The rebuild option for primary keys creates primary keys for tables that have no key and a single unique index.
You can choose to rebuild all primary keys in your model, or select the tables for which you want to rebuild the primary keys.