Tracking what I eat not only gives me an idea of how much calories are consumed during the day (that goes hand to hand with tracking daily activity). It also helps keeping diet in balance to make sure that no important nutrients miss my plate.
It is Important to Not Starve Yourself
I think a lot of people who want to lose weigh (myself included) go through this phase of "fat comes from food, and if I don't eat at all, I will lose it eventually" thinking.
The problem with this is that not only fat comes from food, all these nutrients your body needs to maintain its wellbeing (fat including) come from what you eat.
And all though body can survive w/o important nutrients for some time, after that it will not be able to. In lucky case, you will be back to square one starting it all over again (eating all fat back, filling hungry forever and being miserable), unlucky one may result in getting sick.
It is important to eat and and it is important to eat right. It is also important to know what is too much and what is too less. This is where food trackers come in handy.
Being an internet kind of girl, I use www.myfitnesspal.com to track my food intake.
It integrates well with some of activity trackers so the data gets imported back and forth (that includes FitBit and iHealth, for example); it has web account and mobile app so I can keep track on a go; besides that it has bar code scanner and decent food library. So it works well for me, see my food diary here.
At any given day, I know if I am eating enough of protein, or too much of fat, or need to start reducing amount of sodium or carbohydrates. It also gives me a chance to treat myself: nothing is wrong with a scoop of ice cream or a piece of chocolate every once in a while, if I worked hard and earned these calories with a session of running, elliptical or some other fitness activity.
At times I also consider getting one of these kitchen scales to make my tracking more accurate than "1/4 of tomato of average size per salad". But I didn't get to that point just yet.
I also use MyFitnessPal to make better choices about what I order in restaurants. Even for these of them who labels food with "light" or "lower calorie" options it helps to know how low the "lower calorie" option is. Many of popular restaurant foods are already in the MyFitnessPal library available for search, so decision is easier.
As a summary to this post, I want to repeat myself, because this is important: whatever changes you decide to do with your diet - make them slowly, listen to how your body reacts to these changes (and give it some time to react), stick to whatever works and discard what doesn't, make sure changes are healthy and you accept them as a part of your healthier life from now on. Don't force yourself into changes, or it isn't going to work. Period.