What is the added benefit of using 所 in 我所认识的人?
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


我所认识的人 excludes：


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:


  
  我所认识的人
  我认识的人
  


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.