grammaticality - Why do we say "write in" a journal instead of "write" journal?

Though I have always used and heard the phrase 'Write a journal' only, I believe
'Write in a journal' can be used typically when you are going to write something in your journal. Eg : 'I am going to write this in my journal'.

But when you state that as a hobby or a thing which you do daily then you will say - "I write a journal".