The estimation is more important if the task is from clients. A client cares about the time more than others because he has to pay for having this task done. Thus, he will be happier to see a notification with an estimation. With this estimation, he can stop you easily if he thinks the time is a bit more than excepted. No client would like to know it takes a lot of time to complete this task without being informed of this in advance.

Figure: Bad Example - Send a task to self without an estimation

Figure: Good Example - Send a task to self with an estimation