Are there any spiritual benefits in writing a diary? Question: Are there any spiritual benefits in writing a diary? There have been many saints who have wrote a diary. Is it somehow beneficial? Why? Did any saint recommend it?  

I am asking the question from the Catholic viewpoint.
Yes. 


For comfort. The Bible is filled with calls for people to remember the works that God has performed in the past as a means to sustain faith during present times of trouble.



  I will remember the deeds of the Lord;
      yes, I will remember your miracles of long ago. Psalm 77:11


If you have received blessings via answers to prayer, or the help of another Christian, or found solace in any other way that your faith provides, by writing down these things today, you can help yourself tomorrow, and also enable you to share God's good news with others and comfort them.


For Wisdom. In a lifetime, a faithful Christian will hear thousands of sermons, homilies and exhortations. As you incorporate these teachings of the church into your life and adapt them to your situation, and gain greater insight through practice, you will learn things that it would be tragic to forget. By writing them down and revisiting them from time to time, you will continually add to your knowledge of God and his ways, rather than continually needing to learn the same things over and over again. As Saint James said:



  23 Anyone who listens to the word but does not do what it says is like
  someone who looks at his face in a mirror 24 and, after looking at
  himself, goes away and immediately forgets what he looks like. 25 But
  whoever looks intently into the perfect law that gives freedom, and
  continues in it—not forgetting what they have heard, but doing it—they
  will be blessed in what they do. (James 1:23-25)


A diary will help you not be that person.


For measuring growth. Do you have questions? Doctrines that perplex or offend? Teachings that make no sense? Write down your questions and use them as a springboard for study. As you find satisfactory answers to those questions later in life, write them down. This will safeguard your heart from unbelief, because nagging questions are like a cancer on your soul. And when you see tangible proof that you are growing in understanding as the questions dwindle in number, this will be a great encouragement to your soul.



  11 Now the Berean Jews were of more noble character than those in
  Thessalonica, for they received the message with great eagerness and
  examined the Scriptures every day to see if what Paul said was true.
  (Acts 17:11)



To combat prayerlessness. God never answers my prayers! How do you deal with that thought? You write down your prayers, you revisit them from time to time and renew them, and when you receive an answer, you write that down too. Over time, you will be encouraged to pray more as you see your efforts rewarded. Recall Jesus' parable of the persistent widow:



  18 Then Jesus told his disciples a parable to show them that they
  should always pray and not give up. 2 He said: “In a certain town
  there was a judge who neither feared God nor cared what people
  thought. 3 And there was a widow in that town who kept coming to him
  with the plea, ‘Grant me justice against my adversary.’
  
  4 “For some time he refused. But finally he said to himself, ‘Even
  though I don’t fear God or care what people think, 5 yet because this
  widow keeps bothering me, I will see that she gets justice, so that
  she won’t eventually come and attack me!’”
  
  6 And the Lord said, “Listen to what the unjust judge says. 7 And will
  not God bring about justice for his chosen ones, who cry out to him
  day and night? Will he keep putting them off? 8 I tell you, he will
  see that they get justice, and quickly. However, when the Son of Man
  comes, will he find faith on the earth?” (Luke 18:1-8)


Keep praying. Don't give up!


To promote joy. It is possible to journal in a destructive way, so make sure that you do not neglect thankfulness in what you write. As Paul said:



  8 Finally, brothers and sisters, whatever is true, whatever is noble,
  whatever is right, whatever is pure, whatever is lovely, whatever is
  admirable—if anything is excellent or praiseworthy—think about such
  things. (Philippians 4:8)


Do this and you will invite joy into your life, and journaling can play a part in it.


To mourn in a healthy way. Life is filled with trials. Turning them into poems and hymns that you record in your journal and offering them back to God can help you grieve without regret, without despair. Some of the greatest hymns in Christendom were written by men and women who experienced horrible loss. As Elihu said when comforting Job:

“People cry out under a load of oppression;
    they plead for relief from the arm of the powerful.
10 But no one says, ‘Where is God my Maker,
    who gives songs in the night,
11 who teaches us more than he teaches the beasts of the earth
    and makes us wiser than the birds in the sky?’ (Job 35:9-11)


Write down your own "songs in the night". This will help you pass through your troubles and come out on the other side more whole and healthy.