It can help you to get a reality check of how many calories you are actually eating.
It may help you to identify a problem area so that you can move past a plateau.
Some people may find that this regimented style helps to keep them on track.
Using a computer/internet program to log your food and calories will usually also give you a breakdown of other nutrients that you are ingesting. This could help you to identify areas where you are lacking and areas where you are overdoing it.
Calorie counting can be very time consuming.
It can be too regimented making the process of eating a very negative experience.
It makes you rely more on numbers and less on your natural instincts of hunger and fullness.
Unless you are taking the time to focus on what those calories mean (ie how full/hungry you feel, what the portions look like on your plate, etc), calorie counting will not teach you anything in the long run. However, paying attention to those items can make it a great learning experience.
Does anyone else have any pros and cons they would like to add to the list?