Artificial primary keys in many-to-many relationships

Checking your article "A Dating App Data Model" I saw that in relations that appear from a many-to-many relationship (for instance between relations "user_account" and "gender") you add an artificial primary key (in the case exposed, primary key is a numeric field "id" but the primary key should be the combination of "user_account_id" and "gender_id").

Is there a technical reason to do that?