Customer also need reports with details. While working for long hours, sometime a very small description is added for say 20 hours of work. This is because developers add comments on completion of task and they could not remember all the details.
This Tracker force user to enter comments at the end of each hours and keep a log of it in a very simplified way. Later when task is completed or invoice period reached, user can mail all the logs in excel to customer with detailed comments and time.
This is very easy way of keeping track of work and increasing client trust in your delivery.