Introspecting Emotions in Private
I believe the essential purpose of a journal is to introspect one's emotions in private. Why do that?
Well, first let's agree that introspecting one's emotions is a value. When you introspect, you answer the questions, "what do I feel?" and "why do I feel it?" By virtue of answering these questions, you acknowledge that you feel the way you do, and identify the source.
Emotions are the product of automatized value-judgments. They are messages from your own subconscious databank, which take over your mind until acknowledged. Until you introspect them, in effect saying, "message received," they keep signaling, taking up mental room. Once you've introspected them, the intensity goes down, and you can think about whether the message is important, relevant, and true.
The alternative is to suppress the emotions — which has many short- and long-term bad consequences.
There are many reasons that people choose to explore their emotions in conversation, rather than in private. The most obvious would be that a therapist or friend can give you empathy for the feelings and perhaps help you understand them better.
But there are several reasons for doing it in private:
First of all, you are more likely to accept your feelings 100% than someone who is not a therapist. When you discuss your feelings with another person, there is a great risk of their thinking or saying, "you shouldn't feel that way." There's nothing more alienating — and it stops the introspection process cold.
Second of all, you are the person who needs to understand your emotions, and you are the person with the background knowledge to make sense of them. It is often more efficient to figure out the causes of a feeling yourself, rather than to try to explain the circumstances to another person and deal with their hypotheses.
Finally, even if you have a good friend or therapist who makes a good sounding board, you don't always have the benefit of their time. It's important to be able take care of your emotions when you need to.
The ability to introspect one's emotions is a skill. I believe that regular journaling is the best way to develop that skill over time.
Once you have the ability, you can't imagine how you could live without it. When you can introspect your feelings after every celebration and during every crisis, you gain a self-knowledge, and personal ownership, that is an invaluable aspect of a rich life.