Journals are, or were common items that contained stories that develop the plot of the game. They are worth a read, to know more about the game's plot. But after reading you should discard the journal, because it takes up inventory space.
Journals were added around v1.0.0 and more of them were added in v.3.3.3. They were left by the Journal Writer, scattered around the second map. Journals are not included in the new Kin reimagined map. There are also documents that works the same as journals, though some or all of them were not made by the Journal Writer. Gusmanak stated that they were "designed to shed light on Trinity Corp and what happened in the "Amend" region." in the update log. It can be obtained by exploiting (although not recommended) due to the fact that it is still in the games coding but does not spawn
The Journal Writer is presumably female.(NOT CONFIRMED) They have been staying in Amend before and during the early days of the apocalypse.
There's also more journal writers: Colonel Parsons, Private Limbrick, Dr. Savannah Norton and Dr. Kamran Rehman, two of them are both Trinity Corp. agents and the other two are from the military.