I don't know why I started writing a journal, but when I look back on all the things I felt the need to chronicle, circa 1981, it seems it was mainly to do with the price of food in the Sixth Form College cafe ('sausage roll, 50p; cookie, 35p' etc.) and whether I could make my lunch allowance stretch to the end of the week. It does not make for inspiring reading.
Thankfully more interesting subjects for journalling arose, chief of which was a newly invigorated Christian faith, which needed exploring via prayer topics and reflections on the bible and on events. And so prayer, bible reading and writing became inextricably linked, and I've been going about 30 years now. Hence the large towers of spiral bound notebooks piling up in various cupboards.
I also kept a journal of the first year of each of the children's lives, which they will no doubt find either vastly amusing or deeply embarrassing when I'm gone ('was up feeding four times in the night, three dirty nappies, ran out of clean babygros, feeling tired and fed up'. That kind of thing).
One major benefit of a spiritual journal is the re-reading. Without it I think I'd simply forget the things God was trying to teach me. You get a sense that somehow there's a progression to the spiritual life, even if it's a regression followed by something important learnt. There's something about the physical writing of it that slows your thoughts down and helps you reflect - or to notice patterns to events or feelings, to give thanks for stuff and/or to give you permission to be sad.
I suppose if the writers of the 66 Old and New Testament documents hadn't recorded the happenings of God's people down the centuries, we wouldn't have a bible at all and wouldn't even know anything about Jesus, so you could argue that the discipline of writing is foundational in Christianity. We're a people of the Book.
And, for better or worse, I'm a person of the spiral bound notebook.