Keeping a written record of one's own conduct and thoughts is old, appearing in philosophical and religious traditions long before the modern journal. The older forms were far more structured.
Ancient practice was review rather than expression
Philosophical schools in antiquity recommended a daily review in which a person went over the day's actions, identifying what was done well and what was not.
The exercise was evaluative and repetitive rather than expressive, and it used a fixed set of questions applied each time rather than whatever happened to be pressing.
Written notes in this tradition were often reminders to oneself of principles already accepted, which is a different genre from a record of events or feelings.
Religious traditions formalised the questions
Several traditions developed structured examinations conducted at fixed times, working through categories of conduct in a set order rather than in whatever order they came to mind.
The structure serves a purpose, since unaided recall favours the memorable and the recent while a list reaches material that would otherwise be skipped.
These practices were usually paired with something beyond the writing, whether confession, direction or a resolution for the following day, so the record was not the endpoint.
Writing changes what can be examined
A thought written down becomes an object that can be reread, compared with earlier entries and evaluated at a distance from the state that produced it.
Rereading is where much of the value sits, because a pattern visible across weeks is invisible on any single day, and only a record makes the comparison possible.
Journals kept without ever being reread lose this entirely, which is one reason the older practices specified periodic review as part of the discipline.
Free-form writing works differently
Modern journalling is usually unstructured, and its recommended benefit is the discharge of unexamined material rather than assessment against a standard.
That has its own value, and people frequently report that writing clarifies a difficulty they could not articulate while turning it over silently.
It answers a different question from the older exercise, though, and treating the two as the same practice obscures what each is for.
The risk of rehearsal
Writers in the older traditions warned that dwelling repeatedly on a grievance or a fault can entrench it, turning the record into a rehearsal of the difficulty.
Their usual safeguard was structure: a bounded time, a fixed set of questions and a concluding resolution, all of which limit how long the attention circles.
Where writing consistently makes a person feel worse rather than clearer, that is worth taking seriously, and persistent distress warrants qualified support rather than more pages.