This reminds me of a person I worked with who would wait until the evening to reply to most emails. I assumed this was so at every morning standup they could say they were waiting on someone else to get back on something.
Outlook will display the time "sent" as the time you hit send. Then they receive it at the scheduled time, and will be marked as the "received" time. Two different time stamps.
Scheduling correspondence generally falls into one of a couple categories:
This message contains information you don't want them to have until after an event. Raises, Layoffs, Reorgs
It's 2am and you don't want to bother them at this hour. (more important for Slack)
They're not in our timezone and you want it to hit the box when they're fresh to work on it. You don't want them to start a sprawling project at 4:30 on a Friday.
It's the weekend and there's no reason for them to worry about it until Monday.
I have a guy, he's amazing but he just does that. I have to be very careful to not to send him anything outside of normal work hours because he will spin up and do it and that's not what I want.