Same reason I'd have to estimate how long a given task will take from past experience and current circumstances/condition, some parts may take more or less time than anticipated and the task may complete faster or slower depending on outside circumstances, but I will keep anyone who needs to know informed of my progress and continue to give further estimates if it makes someone feel better, even if it suddenly goes from an hour to a week coz I done my back in or something. It just reassures you that it will be done and nothing has went wrong. It's reassuring to know it's making progress at all!