You spoke knowledgeable about the topics we covered in the interview and demonstrated expertise in the field.
Assists team members in making good hiring decisions. When hiring teams have a well-written document about a candidate interview, they can use it to form important opinions that they wouldn't be able to by just hearing about it. This is also important for hiring teams who have to decide between multiple candidates as the document gives them something to refer to, regardless of how much time has passed since the interview.