That's still for leaving mail on the server so you can respond to it once you get back to your office. Most people don't want to be forced to respond to all emails on a mobile device. They would rather just read the email and leave it on the server so they can check it again when they get to their home or office computer. Leaving it on their for one day is so you can check it that afternoon or night. It doesn't delete the email from your iPhone.