Let just get this straight from the start, food journal is important if you want to lose weight without much headaches and frustration. With that said, for the strong willed it is not a necessity, in fact many people have lost a lot of weight without the use of keeping a food journal.
If you can afford the time to write down everything you eat then you shall benefit greatly from keeping a food journal. It can also act as a good reference if you want to help someone else lose weight (obviously you would do that after you yourself have lost some decent amount of weight) and it can also help you in future if you gain weight by chance (which happens a lot if you take diet as a means to an end).
But there is just nothing better to be able to control what you eat and this plays a significant role in weight loss solutions. Keeping a food journal will force you to look up the amount of calories you are eating daily. The effort (the actual writing on a piece of paper) you would make to record the food items you ate each day will force you to eat less because of two reasons:
It is a general fact that people tend to forget what they ate for the day and hence think that dieting does not work when it comes to losing weight. That is of course not true. Keeping a journal will also make you more realistic about the quantity of food you should be eating in order to lose weight and stay healthy.
Basically a food journal is a written record of every single thing you eat in the entire day. Everything you eat, everything you drink. Even if it has 10 calories, it counts. You record them all in your food journal. At the end of the day you count the number of calories that you have taken in.
And then make the required adjustments to your diet in order to lose the amount of weight you want to lose in a given time period.