Many fitness and nutrition experts suggest using a journal to keep track of exercise and food intake. how can using a journal someone become more fit or change body composition?

Well for example many people have a journal that counts calories they set a certian amount of calories they can intake and evrything they eat they record the calories until they reach the max calorie limit that they have set and if they work out that burns a certain amount of calories so depending on the work out they take the calories they burned from that work out and subtract it from the calories they have already in taken so that they can either eat more calories or need to burn those calories to meet their goal

You track your calories and hold yourself respinsibke

Yes, this is very true. specifically causing damage in the  dentrite.