In Defoe's Robinson Crusoe, I believe that Crusoe starts a journal for much the same reasons that he builds furniture, grows food, improves his "home," and tries to domesticate the goats: he is doing his very best to act as if he were within civilized society so as not to lose his mind with loneliness and worry.
When Crusoe first arrives on the island, his first concern is gathering as many supplies, tools and materials from the wrecked ship as possible. His plan is to do his best to survive until he is rescued. This process takes a long time and requires a great deal of work. After there is nothing else to collect, he begins to build a substantial shelter, manages the supplies (such as separating gunpowder so it doesn't all blow up), makes furniture, finds a source of fresh water, and goes about learning ways to feed himself. These things also take a long time. He eventually learns to make candles so that he does not need to go to sleep when the sun goes down. In essence, Crusoe does his best to create some semblance of a homestead, which not only protects him and adds to his comfort, but also keeps him occupied. When he can no longer do this, he sometimes searches for rescue; seeing no ships on the horizon, he cries. Keeping a journal keeps track of the days, but helps give him new direction as well. He writes:
Some days after this, and after I had been on board the ship, and got all that I could out of her, yet I could not forbear getting up to the top of a little mountain and looking out to sea, in hopes of seeing a ship; then fancy at a vast distance I spied a sail, please myself with the hopes of it, and then after looking steadily, till I was almost blind, lose it quite, and sit down and weep like a child, and thus increase my misery by my folly.
But having gotten over these things in some measure, and having settled my household staff and habitation, made me a table and a chair, and all as handsome about me as I could, I began to keep my journal; of which I shall here give you the copy (though in it will be told all these particulars over again) as long as it lasted; for having no more ink, I was forced to leave it off.
Crusoe is not a strong man when he arrives on the island. He is a member of the comfortable middle class in England. He is in no way prepared for the demands of being stranded alone on a deserted island. In the face of crisis, however, he proves that he has inner- fortitude in rising to each occasion in order to survive. He seems to have a good deal of common sense and he knows himself. It seems that he quickly realizes that attending to his physical comfort is paramount. Once he has that well in hand, his mental and emotional states must also be seen to. The journal helps Crusoe focus on civilized behavior, which keeps him from going insane.