This is an advanced question I think.
所 here loans the sense of 所有(的)（owned by）.
The structure 'pronoun + 所 + verb + 的( + noun)' indicates a subject-predicate-object relationship. In my opinion, this could have some exclusive effect or a bit emphasis on subject-predicate-object relationship. For example,
我所认识的人 // the people I know
- Subject 我， not others like 你，他, and etc.
- Verb 认识， not other verbs like 信任，尊敬, and etc.
- Target 人， not other thing like 东西，事情， and etc.
Compare the two phrases below:
1 shows the preciseness of each component, subject, predicate, and object. 2 is just a casual way to say it. That being said, 1 is used when you are trying to be formal, clear or precise, while 2 is used when you don't have to be that formal and precise.