Why protagonist in Paul Schrader’s movies are always writing a journal?

At regular interval the character is going to discuss his life with a therapist or she’s going to write letter or diary entry but she’s going to rise above the meat and-potato reality of physical world. — Chuck Palahniuk

There are scenes written to let you glimpse into the mind of a character.

Ever watched House Of Cards or Fleabag and wondered why we like it so much when the protagonist stares directly into the camera and talk like it's just you and them.

In those exact moments, they reveal their innermost thoughts that can’t be said — as they are too outrageous, sarcastic, or simply personal — something they won’t or can’t reveal to the world.

Such thoughts are easy to write in a journal or talked with a therapist as there’s some kind of confidentiality involved.

Few techniques storytellers used over the years to show the thought process of a character —

1. Voice Over —

Goodfellas, The Shawshank Redemption, Badlands, The Tree Of Life (Terrence Malick movies)

2. Writing Journal —

Taxi Driver, Mad Men (S04E08), First Reformer (most of Paul Schrader movies)

3. Courtroom Trial / confession—

The Social Network.

4. Talking to the camera / Breaking 4th Wall —

House of Cards, Fleabag, Deadpool, etc.

5. Talking to the therapist —

Good Will Hunting, 50/50, etc.

6. Telling a life story to a stranger or some other character —

Forest Gump, The fall (2006), etc.

7. Writing a letter —

The Notebook, atonement, etc.

The purpose of this scene is to help the audience/reader show the thought process of a character.

The camera has its limitation, it can only track & record a character's moment-by-moment action.

That’s why through voice-over or character writing a letter — you get to hear the character’s commentary on what’s happening. It gives us the philosophical meaning, satirical anecdote, hidden backstory, or at least the characters' subjective interpretation of the events.

They may even be forced to talk about these things like a courtroom scene in The Social Network where protagonist mark is confronted to talk about the dark past of how and why he betrayed his best friend and may have stolen a billion-dollar idea that became Facebook.

Switching to these scenes for a short interval of time can also give you a buffer between the two scenes. It can be your way of transitioning from one scene to another, which will allow you to imply the passing of time.