If you have the relationship setup in your contact, you can tell Siri to call your wife/sister/younger cousin etc.
Siri will also set up the relationship automatically if you tell her to “call my wife”, and don’t have that relationship set - she will ask who your wife is, update the contact, and make the call.
It’s pretty neat if your family names are hard for (English) Siri to understand, because you can just “hey siri call my wife” instead of trying to “hey siri call <mangled mispronounced name of my wife that might get Siri to pick up the name correctly>”.
What if I want to call my boss? Or my hairdresser? Or my hairdresser's uncle? If Siri can do speech recognition, can't it compare the result to a generic text field?