I journal to my t outside of session also, but not every time. I think it can be helpful to address some stuff that way (or at least bring it up). I'm finding tho (at least for me), I then forget what I had mentioned i wanted to talk about...
I think maybe she's just trying to bring focus to the work. Then, if there's something you definitely need to address, you will bring it up in season rather than just reply on the journaling. I've noticed I step back and let my t pick what she thinks is important to address when I write it all down. I end up missing the chance to talk about stuff I really want to cover. For myself, I've decided I want to read her the latest entry so I can focus on what I feel really needs it... I don't know if you tend to do the same thing and she's trying to give you more immediate control over the season that way?