What is the advantage of Foreign Key as Primary Key?

Question

Database design

Since we are talking about RDB,
Doctrine 2 Docs

is used as an example. Everything is great, but apart from reducing the sampling speed, I do not see any obvious advantages.

For example, there is a User entity and it has UserDetails.
User (password, email, id, nickname), UserDetails (bornAt, firstName, lastName, adress, description....)

Now I use the usual approach with Natural key surrogate key aka simple surrogate key