How is this a feature worth developing? Is leaving the email unread until you want to read it too difficult? I can't think of a single use case for this.
Once you see the inbox as a task list, you will understand the feature.
A to-do list sorted by the time the task was put on the list is not optimal. Some people would like to see the tasks that they have to do this week (send pictures to Emily), and not in three months (check-in for the upcoming flight).
I was not aware of this feature myself until just now, but I'm going to be a user of it.
You can also snooze to locations. So having the email popup when you get home for example.
I can easily keep Inbox:0 by deferring things until I actually need to deal with them. Inbox does the best job by bundling like emails which can then processed together.