It helps people who are thinking, "I have two minutes, can I deal with this now?"
In the example the answer would be no, because I'm expecting a response, and just reading my email would take all that, so there is no time to write a response.
I am thinking: "This block of text looks like it will take me 4 minutes. Do I have 4 minutes"?
The estimate of someone else about how long something will take me to digest and work through may be completely irrelevan to me.
Things someone with a finance rather than technical background thinks will take 10 minutes, I may be able to quickscan and reply to within less than a minute. On the other hand, I have had questions to POs about mails regarding business minutia that took time for me to roughly grasp, and they replied within a few minutes, because that's their area of expertise.
