db_datawriter vs grant delete, update, insert
You say "It seems that every table in the database is listed here and they all have update, delete, insert, and select".  If you have not explicitly verified that insert, update, and delete permissions have been provided on every object via this role; these permissions may be missing on some objects.

The role [db_datawriter] guarantees that insert, update, and delete permissions are provided on all objects within the database.  This also includes any future objects to be created.