What are the benefits of a bookkeeping habit?

Listen to me tell you a story to see if it helps. When I was in junior high school, I boarded at my teacher's house, and my roommate was a little fat kid. He liked to come up with new ideas, and bookkeeping was one of them. At that time, I thought, "What's the point of keeping track of all this? You only have so much pocket money a week, how can you keep track of the money? Later I realized that I was wrong, and it was really possible. The little fat man just started to keep track of the time, more detailed than the running account, buy snacks one by one to remember, even the brand of chili bars are remembered, this is how much pain. But he can't immediately pull out a small notebook to keep track of every purchase, and sometimes he forgets. Therefore, he has learned to keep track of what he is going to buy later, and then write it down first and then add it when he comes back. One day, I went to the supermarket with him, he didn't have enough money to buy something, so he borrowed 20 from me, and when he came back, he started to write down what he had bought, and he also wrote down that he had returned 20 to me, which I saw and said, "You didn't pay me back". He said he would return it tomorrow, and he would write it down first. As a result, after more than a week, he suddenly brought up said that also my money did not, I TM how to remember, if not he brought up, I can not remember this, I think back, still can not remember to return. At this time, he took out a small notebook while happily said "look at the account book will know. All of a sudden, as if I opened the two chakras, as in the dark road ahead of a bright light Yes, my memory all recovered, but no use, only to see him turn to the page, read aloud to the "pay back money twenty". And then reverently closed, whispered to me that the money has been returned.