You're right, it's up to you to decide when you read it. All I'm saying is that I have a right, in a commercial environment, to know that you have done so. Frankly, I would not usually care or take note of when you happened to read it, all I care about is that you have, and that I can therefore move onto another task.

An example. A colleague emails a request for information. I respond. He then later claims to have not received the information. A read-receipt is evidence that this is not the case. I have no problem with you ignoring it, but claiming to have not received it is just dishonest. Something that has no place in a business setting.