Why is it important to identify the source of a subject’s emotion?
Since you have tagged this with “crime scene investigation” and “interrogation,” I assume that you are asking about the emotions shown by the subject of an interrogation. It is extremely important to identify the sources of your subjects’ emotions because that knowledge can help you to understand what has happened in regard to a given crime.
For example, let us say that you are investigating someone that you see as a suspect. He consistently appears to be emotionally shaken when he sees a particular piece of evidence. This may make you suspect that he knows how that evidence came to be at the crime scene. However, it is very important that you learn whether this truly is the reason why he is reacting to the evidence. You do not want to simply make assumptions without actually ascertaining whether there is any basis for those assumptions.
The same could be true of a witness who seems nervous. Is the witness nervous because she is really the guilty party? Is she nervous because she is trying to cover for someone? Or is she nervous because she is afraid that the suspects will find out that she has talked to you and will get revenge? The answer to this is clearly important for how you interpret this witness’s testimony.